System.NullReferenceException: "Object reference not set to an instance of an object."

using Microsoft.EntityFrameworkCore;
using Programme.net.Class;
using System.Configuration;
using Action = Programme.net.Class.Action;

namespace Programme.net
{
    public class MobileContext : DbContext
    {

        public virtual DbSet<Action> ActionAll { get; set; }
        public virtual DbSet<User> User { get; set; }
        public virtual DbSet<ButtonGen> ButtonGen { get; set; }
        public virtual DbSet<Question> Question { get; set; }
        public virtual DbSet<Test> Test { get; set; }
        public virtual DbSet<Subdivision> Subdivision { get; set; }
        public virtual DbSet<Library> Library { get; set; }
        public virtual DbSet<Exercises> Exercises { get; set; }
        public virtual DbSet<StateTest> StateTest { get; set; }
        public virtual DbSet<Normativ> Normativ { get; set; }
        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {

            //optionsBuilder.UseSqlServer UseJet ConfigurationManager.ConnectionStrings["Connect"].ConnectionString
            optionsBuilder.UseSqlServer(ConfigurationManager.ConnectionStrings["Connect"].ConnectionString);
            base.OnConfiguring(optionsBuilder);

        }

    }
}