PyQT5. Открытие файла в textEdition в дочернем окне

Я попробовал что-то сделать для вас, скопируйте мой пример и попробуйте. Если что-то будут не понятно - спросите.

В один ответ не помещаюсь - разбиваю на части.

1-я часть модуль design_q1363451.py

from PyQt5 import  QtGui, QtWidgets, QtCore


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.setWindowModality(QtCore.Qt.NonModal)
        MainWindow.setEnabled(True)
        MainWindow.resize(1012, 729)
        sizePolicy = QtWidgets.QSizePolicy(
            QtWidgets.QSizePolicy.Fixed, QtWidgets.QSizePolicy.Fixed)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(MainWindow.sizePolicy().hasHeightForWidth())
        MainWindow.setSizePolicy(sizePolicy)
        MainWindow.setMinimumSize(QtCore.QSize(1012, 570))
        font = QtGui.QFont()
        font.setStyleStrategy(QtGui.QFont.PreferDefault)
        MainWindow.setFont(font)
        MainWindow.setCursor(QtGui.QCursor(QtCore.Qt.ArrowCursor))
        MainWindow.setMouseTracking(False)
        MainWindow.setWindowOpacity(1.0)
        MainWindow.setLayoutDirection(QtCore.Qt.LeftToRight)
        MainWindow.setAutoFillBackground(False)
        MainWindow.setStyleSheet("background-color:#fa8072")
        MainWindow.setInputMethodHints(QtCore.Qt.ImhNone)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.gridLayout_2 = QtWidgets.QGridLayout(self.centralwidget)
        self.gridLayout_2.setObjectName("gridLayout_2")
        self.widget = QtWidgets.QWidget(self.centralwidget)
        self.widget.setMinimumSize(QtCore.QSize(990, 301))
        self.widget.setToolTipDuration(-1)
        self.widget.setLayoutDirection(QtCore.Qt.LeftToRight)
        self.widget.setStyleSheet("background-color:#22222e;\n"
"border-radius:15px;\n"
"")
        self.widget.setObjectName("widget")
        self.gridLayout = QtWidgets.QGridLayout(self.widget)
        self.gridLayout.setContentsMargins(11, 11, 11, 11)
        self.gridLayout.setHorizontalSpacing(7)
        self.gridLayout.setObjectName("gridLayout")
        self.m_field = QtWidgets.QLineEdit(self.widget)
        self.m_field.setStyleSheet("background-color:#ffe4e1;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:black;")
        self.m_field.setObjectName("m_field")
        self.gridLayout.addWidget(self.m_field, 4, 2, 1, 1)
        self.label_7 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe Bold")
        font.setPointSize(14)
        font.setStyleStrategy(QtGui.QFont.PreferDefault)
        self.label_7.setFont(font)
        self.label_7.setStyleSheet("color:white;")
        self.label_7.setObjectName("label_7")
        self.gridLayout.addWidget(self.label_7, 2, 0, 1, 1)
        self.label_2 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe Bold")
        font.setPointSize(14)
        font.setStyleStrategy(QtGui.QFont.PreferDefault)
        self.label_2.setFont(font)
        self.label_2.setStyleSheet("color:white;")
        self.label_2.setObjectName("label_2")
        self.gridLayout.addWidget(self.label_2, 0, 0, 1, 1)
        self.label_5 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe Bold")
        font.setPointSize(14)
        font.setStyleStrategy(QtGui.QFont.PreferDefault)
        self.label_5.setFont(font)
        self.label_5.setStyleSheet("color:white;")
        self.label_5.setObjectName("label_5")
        self.gridLayout.addWidget(self.label_5, 1, 0, 1, 1)
        self.label = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setPointSize(14)
        self.label.setFont(font)
        self.label.setStyleSheet("color:white;")
        self.label.setObjectName("label")
        self.gridLayout.addWidget(self.label, 3, 0, 1, 1)
        self.class_bt = QtWidgets.QComboBox(self.widget)
        self.class_bt.setStyleSheet("background-color:#ffe4e1;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:black;")
        self.class_bt.setObjectName("class_bt")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.class_bt.addItem("")
        self.gridLayout.addWidget(self.class_bt, 5, 2, 1, 1)
        self.label_13 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setPointSize(14)
        font.setBold(True)
        font.setWeight(75)
        self.label_13.setFont(font)
        self.label_13.setStyleSheet("color:white")
        self.label_13.setObjectName("label_13")
        self.gridLayout.addWidget(self.label_13, 5, 0, 1, 1)
        self.label_10 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setPointSize(14)
        self.label_10.setFont(font)
        self.label_10.setStyleSheet("color:white;")
        self.label_10.setObjectName("label_10")
        self.gridLayout.addWidget(self.label_10, 4, 0, 1, 1)
        self.h_field = QtWidgets.QLineEdit(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe")
        self.h_field.setFont(font)
        self.h_field.setStyleSheet("background-color:#ffe4e1;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:black;")
        self.h_field.setText("")
        self.h_field.setObjectName("h_field")
        self.gridLayout.addWidget(self.h_field, 2, 2, 1, 1)
        self.label_4 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe Bold")
        font.setPointSize(14)
        font.setStyleStrategy(QtGui.QFont.PreferDefault)
        self.label_4.setFont(font)
        self.label_4.setStyleSheet("color:white;")
        self.label_4.setObjectName("label_4")
        self.gridLayout.addWidget(self.label_4, 2, 3, 1, 1)
        self.label_11 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setPointSize(14)
        self.label_11.setFont(font)
        self.label_11.setStyleSheet("color:white;")
        self.label_11.setObjectName("label_11")
        self.gridLayout.addWidget(self.label_11, 3, 3, 1, 1)
        self.pushButton = QtWidgets.QPushButton(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe Bold")
        font.setPointSize(17)
        font.setStyleStrategy(QtGui.QFont.PreferDefault)
        self.pushButton.setFont(font)
        self.pushButton.setStyleSheet("QPushButton{\n"
"background-color:#fa8072;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:white;\n"
"}\n"
"\n"
"QPushButton:pressed {\n"
"background-color:#fa4244;\n"
"}")
        self.pushButton.setObjectName("pushButton")
        self.gridLayout.addWidget(self.pushButton, 7, 2, 1, 1)
        self.a_field = QtWidgets.QLineEdit(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe")
        self.a_field.setFont(font)
        self.a_field.setCursor(QtGui.QCursor(QtCore.Qt.PointingHandCursor))
        self.a_field.setAcceptDrops(True)
        self.a_field.setAutoFillBackground(False)
        self.a_field.setStyleSheet("background-color:#ffe4e1;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:black;\n"
"\n"
"\n"
"")
        self.a_field.setText("")
        self.a_field.setFrame(True)
        self.a_field.setEchoMode(QtWidgets.QLineEdit.Normal)
        self.a_field.setDragEnabled(False)
        self.a_field.setReadOnly(False)
        self.a_field.setObjectName("a_field")
        self.gridLayout.addWidget(self.a_field, 0, 2, 1, 1)
        self.as_field = QtWidgets.QLineEdit(self.widget)
        self.as_field.setStyleSheet("background-color:#ffe4e1;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:black;")
        self.as_field.setObjectName("as_field")
        self.gridLayout.addWidget(self.as_field, 3, 2, 1, 1)
        self.label_12 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setPointSize(12)
        self.label_12.setFont(font)
        self.label_12.setStyleSheet("color:white;")
        self.label_12.setObjectName("label_12")
        self.gridLayout.addWidget(self.label_12, 4, 3, 1, 1)
        self.b_field = QtWidgets.QLineEdit(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe")
        self.b_field.setFont(font)
        self.b_field.setStyleSheet("background-color:#ffe4e1;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:black;")
        self.b_field.setText("")
        self.b_field.setObjectName("b_field")
        self.gridLayout.addWidget(self.b_field, 1, 2, 1, 1)
        self.label_6 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe Bold")
        font.setPointSize(14)
        font.setStyleStrategy(QtGui.QFont.PreferDefault)
        self.label_6.setFont(font)
        self.label_6.setStyleSheet("color:white;")
        self.label_6.setObjectName("label_6")
        self.gridLayout.addWidget(self.label_6, 0, 3, 1, 1)
        self.label_3 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setFamily("Circe Bold")
        font.setPointSize(14)
        font.setStyleStrategy(QtGui.QFont.PreferDefault)
        self.label_3.setFont(font)
        self.label_3.setStyleSheet("color:white;")
        self.label_3.setObjectName("label_3")
        self.gridLayout.addWidget(self.label_3, 1, 3, 1, 1)
        self.class_ar = QtWidgets.QComboBox(self.widget)
        self.class_ar.setStyleSheet("background-color:#ffe4e1;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:black;")
        self.class_ar.setObjectName("class_ar")
        self.class_ar.addItem("")
        self.class_ar.addItem("")
        self.class_ar.addItem("")
        self.class_ar.addItem("")
        self.class_ar.addItem("")
        self.class_ar.addItem("")
        self.class_ar.addItem("")
        self.gridLayout.addWidget(self.class_ar, 6, 2, 1, 1)
        self.label_14 = QtWidgets.QLabel(self.widget)
        font = QtGui.QFont()
        font.setPointSize(14)
        font.setBold(True)
        font.setWeight(75)
        self.label_14.setFont(font)
        self.label_14.setStyleSheet("color:white;")
        self.label_14.setObjectName("label_14")
        self.gridLayout.addWidget(self.label_14, 6, 0, 1, 1)
        self.gridLayout_2.addWidget(self.widget, 3, 0, 1, 4)
        self.label_9 = QtWidgets.QLabel(self.centralwidget)
        self.label_9.setStyleSheet("")
        self.label_9.setText("")
        
# !!!        self.label_9.setPixmap(QtGui.QPixmap(":/newPrefix/123434.png"))
        self.label_9.setPixmap(QtGui.QPixmap("Ok.png"))            # установите свое    
        
        self.label_9.setObjectName("label_9")
        self.gridLayout_2.addWidget(self.label_9, 1, 1, 1, 1)
        self.answer_text = QtWidgets.QTextBrowser(self.centralwidget)
        self.answer_text.setStyleSheet("background-color:#ffe4e1;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:black;")
        self.answer_text.setObjectName("answer_text")
        self.gridLayout_2.addWidget(self.answer_text, 1, 3, 1, 1)
        self.task_text = QtWidgets.QTextBrowser(self.centralwidget)
        sizePolicy = QtWidgets.QSizePolicy(QtWidgets.QSizePolicy.Preferred, QtWidgets.QSizePolicy.Preferred)
        sizePolicy.setHorizontalStretch(0)
        sizePolicy.setVerticalStretch(0)
        sizePolicy.setHeightForWidth(self.task_text.sizePolicy().hasHeightForWidth())
        self.task_text.setSizePolicy(sizePolicy)
        self.task_text.setStyleSheet("background-color:#ffe4e1;\n"
"border: 2px solid #fa8072;\n"
"border-radius: 10px;\n"
"color:black;")
        self.task_text.setObjectName("task_text")
        self.gridLayout_2.addWidget(self.task_text, 1, 0, 1, 1)
        self.label_8 = QtWidgets.QLabel(self.centralwidget)
        font = QtGui.QFont()
        font.setFamily("Circe ExtraBold")
        font.setPointSize(15)
        font.setStyleStrategy(QtGui.QFont.PreferDefault)
        self.label_8.setFont(font)
        self.label_8.setStyleSheet("color:white;")
        self.label_8.setObjectName("label_8")
        self.gridLayout_2.addWidget(self.label_8, 0, 3, 1, 1)
        MainWindow.setCentralWidget(self.centralwidget)
        self.menuBar = QtWidgets.QMenuBar(MainWindow)
        self.menuBar.setGeometry(QtCore.QRect(0, 0, 1012, 26))
        self.menuBar.setObjectName("menuBar")
        self.menuFile = QtWidgets.QMenu(self.menuBar)
        self.menuFile.setObjectName("menuFile")
        self.menuSound = QtWidgets.QMenu(self.menuFile)
        self.menuSound.setObjectName("menuSound")
        self.menu_Help = QtWidgets.QMenu(self.menuBar)
        self.menu_Help.setObjectName("menu_Help")
        MainWindow.setMenuBar(self.menuBar)
        self.action_Help = QtWidgets.QAction(MainWindow)
        self.action_Help.setObjectName("action_Help")
        self.action_Save = QtWidgets.QAction(MainWindow)
        self.action_Save.setObjectName("action_Save")
        self.action_Quit = QtWidgets.QAction(MainWindow)
        self.action_Quit.setObjectName("action_Quit")
        self.action_Open = QtWidgets.QAction(MainWindow)
        self.action_Open.setObjectName("action_Open")
        self.action_Document = QtWidgets.QAction(MainWindow)
        self.action_Document.setObjectName("action_Document")
        self.actionOn = QtWidgets.QAction(MainWindow)
        self.actionOn.setObjectName("actionOn")
        self.actionOff = QtWidgets.QAction(MainWindow)
        self.actionOff.setObjectName("actionOff")
        self.actionSave_as = QtWidgets.QAction(MainWindow)
        self.actionSave_as.setObjectName("actionSave_as")
        self.menuSound.addAction(self.actionOn)
        self.menuSound.addAction(self.actionOff)
        self.menuFile.addAction(self.menuSound.menuAction())
        self.menuFile.addAction(self.action_Save)
        self.menuFile.addAction(self.actionSave_as)
        self.menuFile.addAction(self.action_Open)
        self.menuFile.addSeparator()
        self.menuFile.addAction(self.action_Quit)
        self.menu_Help.addAction(self.action_Document)
        self.menuBar.addAction(self.menuFile.menuAction())
        self.menuBar.addAction(self.menu_Help.menuAction())

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "Проверка арматуры на прочность"))
        self.m_field.setPlaceholderText(_translate("MainWindow", "550"))
        self.label_7.setText(_translate("MainWindow", "h = "))
        self.label_2.setText(_translate("MainWindow", "a = "))
        self.label_5.setText(_translate("MainWindow", "b = "))
        self.label.setText(_translate("MainWindow", "As="))
        self.class_bt.setItemText(0, _translate("MainWindow", "B10"))
        self.class_bt.setItemText(1, _translate("MainWindow", "B15"))
        self.class_bt.setItemText(2, _translate("MainWindow", "B20"))
        self.class_bt.setItemText(3, _translate("MainWindow", "B25"))
        self.class_bt.setItemText(4, _translate("MainWindow", "B30"))
        self.class_bt.setItemText(5, _translate("MainWindow", "B35"))
        self.class_bt.setItemText(6, _translate("MainWindow", "B40"))
        self.class_bt.setItemText(7, _translate("MainWindow", "B45"))
        self.class_bt.setItemText(8, _translate("MainWindow", "B50"))
        self.class_bt.setItemText(9, _translate("MainWindow", "B55"))
        self.class_bt.setItemText(10, _translate("MainWindow", "B60"))
        self.label_13.setText(_translate("MainWindow", "Класс бетона:"))
        self.label_10.setText(_translate("MainWindow", "M="))
        self.h_field.setPlaceholderText(_translate("MainWindow", "800"))
        self.label_4.setText(_translate("MainWindow", "мм"))
        self.label_11.setText(_translate("MainWindow", "мм2"))
        self.pushButton.setText(_translate("MainWindow", "Рассчитать"))
        self.a_field.setPlaceholderText(_translate("MainWindow", "70"))
        self.as_field.setPlaceholderText(_translate("MainWindow", "2945"))
        self.label_12.setText(_translate("MainWindow", "kH*M"))
        self.b_field.setPlaceholderText(_translate("MainWindow", "300"))
        self.label_6.setText(_translate("MainWindow", "мм"))
        self.label_3.setText(_translate("MainWindow", "мм"))
        self.class_ar.setItemText(0, _translate("MainWindow", "А240"))
        self.class_ar.setItemText(1, _translate("MainWindow", "А400"))
        self.class_ar.setItemText(2, _translate("MainWindow", "А500"))
        self.class_ar.setItemText(3, _translate("MainWindow", "A500СП"))
        self.class_ar.setItemText(4, _translate("MainWindow", "А600"))
        self.class_ar.setItemText(5, _translate("MainWindow", "Ан600С"))
        self.class_ar.setItemText(6, _translate("MainWindow", "В500"))
        self.label_14.setText(_translate("MainWindow", "Класс арматуры:"))
        self.answer_text.setHtml(_translate("MainWindow", "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0//EN\" \"http://www.w3.org/TR/REC-html40/strict.dtd\">\n"
"<html><head><meta name=\"qrichtext\" content=\"1\" /><style type=\"text/css\">\n"
"p, li { white-space: pre-wrap; }\n"
"</style></head><body style=\" font-family:\'MS Shell Dlg 2\'; font-size:7.8pt; font-weight:400; font-style:normal;\">\n"
"<p style=\"-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; font-size:8pt;\"><br /></p></body></html>"))
        self.task_text.setHtml(_translate("MainWindow", "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0//EN\" \"http://www.w3.org/TR/REC-html40/strict.dtd\">\n"
"<html><head><meta name=\"qrichtext\" content=\"1\" /><style type=\"text/css\">\n"
"p, li { white-space: pre-wrap; }\n"
"</style></head><body style=\" font-family:\'MS Shell Dlg 2\'; font-size:7.8pt; font-weight:400; font-style:normal;\">\n"
"<p style=\" margin-top:12px; margin-bottom:12px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px;\"><span style=\" font-family:\'Circe\'; font-size:14pt;\">Дано сечение размерами b, h, a мм; растянутая арматура класса </span><span style=\" font-family:\'Circe\'; font-size:14pt; font-weight:600;\">A400</span><span style=\" font-family:\'Circe\'; font-size:14pt;\"> (Rs = 350 МПа); площадь ее сечения A</span><span style=\" font-family:\'Circe\'; font-size:14pt; vertical-align:sub;\">s</span><span style=\" font-family:\'Circe\'; font-size:14pt;\"> = 2945 мм</span><span style=\" font-family:\'Circe\'; font-size:14pt; vertical-align:super;\">2</span><span style=\" font-family:\'Circe\'; font-size:14pt;\"> (6Ø25); бетон тяжелый класса </span><span style=\" font-family:\'Circe\'; font-size:14pt; font-weight:600;\">B25</span><span style=\" font-family:\'Circe\'; font-size:14pt;\"> (Rb = 14,5 МПа); изгибающий момент с учетом кратковременных нагрузок M = 550 кН•м.<br /></span><span style=\" font-family:\'Circe\'; font-size:14pt; text-decoration: underline;\">Требуется проверить прочность сечения.</span></p></body></html>"))
        self.label_8.setText(_translate("MainWindow", "Прочность арматуры"))
        self.menuFile.setTitle(_translate("MainWindow", "&File"))
        self.menuSound.setTitle(_translate("MainWindow", "Sound"))
        self.menu_Help.setTitle(_translate("MainWindow", "&Help"))
        self.action_Help.setText(_translate("MainWindow", "&Help"))
        self.action_Save.setText(_translate("MainWindow", "&Save"))
        self.action_Quit.setText(_translate("MainWindow", "&Quit"))
        self.action_Open.setText(_translate("MainWindow", "&Open"))
        self.action_Document.setText(_translate("MainWindow", "&Document"))
        self.actionOn.setText(_translate("MainWindow", "On"))
        self.actionOff.setText(_translate("MainWindow", "Off"))
        self.actionSave_as.setText(_translate("MainWindow", "Save as"))
        
# import icon_cwt

2-я часть, модуль main.py

import sys
# ??? from typing import List

from PyQt5 import  QtGui, QtWidgets, QtCore
from PyQt5.Qt import *

from design_q1363451 import Ui_MainWindow
        

#Дочернее окно 1: help.py
#import Help
class Ui_Help(object):
    def setupUi(self, Help):
        Help.setObjectName("Help")
        Help.resize(200, 600)
        self.formLayout = QtWidgets.QFormLayout(Help)
        self.formLayout.setContentsMargins(0, 0, 0, 0)
        self.formLayout.setHorizontalSpacing(0)
        self.formLayout.setObjectName("formLayout")
        self.listWidget = QtWidgets.QListWidget(Help)
        self.listWidget.setEnabled(True)
        self.listWidget.setMinimumSize(QtCore.QSize(200, 600))
        self.listWidget.setResizeMode(QtWidgets.QListView.Fixed)
        self.listWidget.setObjectName("listWidget")
        self.formLayout.setWidget(0, QtWidgets.QFormLayout.FieldRole, self.listWidget)

        self.retranslateUi(Help)
        QtCore.QMetaObject.connectSlotsByName(Help)

    def retranslateUi(self, Help):
        _translate = QtCore.QCoreApplication.translate
        Help.setWindowTitle(_translate("Help", "Helped Dialog"))
        
        
#Дочерние окно 2 в которое надо переносить данные из txt: text.py
#import text
class Ui_Text(object):
    def setupUi(self, Text):
        Text.setObjectName("Text")
        Text.resize(1200, 400)
        Text.setIconSize(QtCore.QSize(30, 30))
        self.centralwidget = QtWidgets.QWidget(Text)
        self.centralwidget.setMinimumSize(QtCore.QSize(500, 400))
        self.centralwidget.setObjectName("centralwidget")
        self.gridLayout = QtWidgets.QGridLayout(self.centralwidget)
        self.gridLayout.setContentsMargins(0, 0, 0, 0)
        self.gridLayout.setSpacing(7)
        self.gridLayout.setObjectName("gridLayout")
        self.textEdit1 = QtWidgets.QTextEdit(self.centralwidget)
        self.textEdit1.setObjectName("textEdit1")
        self.textEdit1.setText('')
        
# ???        text = self.textEdit1
        
        self.gridLayout.addWidget(self.textEdit1, 0, 0, 1, 1)
        Text.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(Text)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 958, 26))
        self.menubar.setObjectName("menubar")
        Text.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(Text)
        self.statusbar.setObjectName("statusbar")
        Text.setStatusBar(self.statusbar)

        self.retranslateUi(Text)
        QtCore.QMetaObject.connectSlotsByName(Text)

    def retranslateUi(self, Text):
        _translate = QtCore.QCoreApplication.translate
        Text.setWindowTitle(_translate("Text", "Ui_Text MainWindow"))
    

#И файл app.py где я инициализирую интерфейс и т.д.
#class Text(QMainWindow,text.Ui_Text):
class Text(QMainWindow, Ui_Text):
    def __init__(self, parent=None):
        """Метод инициализации интерфейса."""
        super(Text, self).__init__(parent) 
        self.setupUi(self)
# ???        self.show()


#class Helped(QDialog, Help.Ui_Help):
class Helped(QDialog, Ui_Help):
    def __init__(self, parent=None):
        """Метод инициализации интерфейса."""
        super(Helped, self).__init__(parent)
        self.setupUi(self)
        
        self.setStyleSheet("background-color:#E79E4F;")             # +++
        self.listWidget.addItems(["Item 1", "Item 2", "Item 3"])    # +++ 
# ???        self.show()


#class ExampleApp(QMainWindow, design.Ui_MainWindow):
class ExampleApp(QMainWindow, Ui_MainWindow):
    def __init__(self):
        super().__init__()
        
        QFontDatabase.addApplicationFont('fonts/circe.ttf')
        QFontDatabase.addApplicationFont('fonts/circe-bold.ttf')
        QFontDatabase.addApplicationFont('fonts/circe-extrabold.ttf')
        self.setupUi(self)
# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv
        self.win = Text()     
        self.okno = Helped()     
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

# ??? count
        self.pushButton.clicked.connect(self.count)
        self.setWindowIcon(QIcon('calculator.png'))
        self.action_Quit.triggered.connect(self.close)
        self.menuFile.triggered.connect(self.selected)

#        self.action_Document.triggered.connect(self.dialog)
        self.action_Document.triggered.connect(self._dialog)
        
        self.action_Save.triggered.connect(self.save)
        self.actionSave_as.triggered.connect(self.save_as)
        
# !!!                                                     vv <---- это не надо       
#        self.action_Open.triggered.connect(self.file_open())
        self.action_Open.triggered.connect(self.file_open)

        self.Sound = QSound('sound_button.wav', self)
        self.pushButton.clicked.connect(self.Sound.play)
        self.action_Quit.triggered.connect(self.Sound.play)
        self.menuFile.triggered.connect(self.Sound.play)
        self.action_Document.triggered.connect(self.Sound.play)
        self.menu_Help.triggered.connect(self.Sound.play)
# ???        self.show()

        self.name = "q1363451_save.txt"                        # !!! "q1363451_save.txt"
     
# -       self.win = Text()

    def okno2(self):
# -       self.okno = Helped()                  
        self.okno.show()

    def file_open(self):
            fname = QFileDialog.getOpenFileName(self, 'Open file', '', '*.txt')[0]
# ???            self.win = text.Ui_Text()
# ???            if len(fname) > 0:
            if fname:                                                  # +++
# !!!                                 vvvvvvvvvvvvvvvv
                with open(fname, 'r', encoding='utf-8') as f:
# -               f = open(fname, 'r')
# -               with f:
                    text1 = f.read()
                    self.win.textEdit1.setText(text1)
                    
                    self.win.show()                                    # +++
# ???       vvvvvvvvvvvv  <---- это зачем  ???                    
            self.okno2()

    def save_as(self):
        self.name = QtWidgets.QFileDialog.getSaveFileName(
            self, 'Save File', '', '(*.txt')[0]
        self.save()

    def save(self):
        a_field = (
            self.a_field.text(), 
            self.b_field.text(), 
            self.h_field.text(), 
            self.as_field.text(), 
            self.m_field.text(),
            self.class_bt.currentText(), 
            self.class_ar.currentText(), 
            
# -           self.answer_text.toPlainText()
# !!!                        vvvvvvvv       <----   # !!!
            self.answer_text.toHtml()               # +++
        )
        #print(a_field, sep='\n') 
        
# -       if a_field[0] == '' or a_field[1] == '' or a_field[2] == '' or a_field[3] == '' or a_field[4] == ''or a_field[7] == '':
# !!! vvv
        if not a_field[0] or not a_field[1] or not a_field[2] or \
           not a_field[3] or not a_field[4] or not a_field[7]:
        
            reply = QtWidgets.QMessageBox.information(
                None,                                    
                "Save", 
                "Enter the data click the calculate button to save the data",
                QtWidgets.QMessageBox.Yes
            )
        else:
            if self.name:
# +++                                     vvvvvvvvvvvvvvvv  
                with open(self.name, 'a', encoding='utf-8') as f:
                    f.write('а= '+a_field[0]+' мм'+'\n'+'b= '+a_field[1] + \
                    ' мм'+'\n'+'h= '+ a_field[2]+' мм'+'\n'+'As= '+ a_field[3]+\
                    ' мм2'+'\n'+'M= '+ a_field[4]+' kH*M'+'\n'+'Класс бетона: '+ \
                    a_field[5]+'\n'+'Класс арматуры: '+ a_field[6]+'\n'+\
                    'Прочность арматуры: '+ a_field[7]+'\n \n')

    def _dialog(self):
# ???
#        dialog = QtWidgets.QDialog()
#        dialog.ui = Help.Ui_Help()
#        dialog.ui.setupUi(dialog)
#        dialog.exec_()
#        dialog.show()

        self.okno.show()                                          # +++

# +++ vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv       
    def count(self):
        pass
        
    def selected(self):
        pass

    def closeEvent(self, event):
        reply = QMessageBox.question(
            None, 
            'Информация',
            "Вы уверены, что хотите уйти?",
             QMessageBox.Yes, QMessageBox.No
         )             
        if reply == QMessageBox.Yes:
            self.okno.close()
            self.win.close()
            event.accept()
            
        else:
            event.ignore()
# +++ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        
        
if __name__ == "__main__":
    app = QtWidgets.QApplication(sys.argv)
    w = ExampleApp()
    w.resize(700, 400)
    w.show()
    sys.exit(app.exec_())