object reference not set to an instance of an object как исправить

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerController : MonoBehaviour
{
    public float speed = 5f;

    private Rigidbody2D rb;
    Vector2 move;


    void FixedUpdate()
    {
        move.x = Input.GetAxisRaw("Horizontal");
        move.y = Input.GetAxisRaw("Vertical");
        rb.MovePosition(rb.position + move * speed * Time.fixedDeltaTime);
    }

}