Error in unity (c#), what to do?

Question

C++ / C#

Good afternoon.
I ran into a problem that I can’t solve in any way.
In unity, I create a 2d game (tds) and when compiling this code

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

public class PlayerController : MonoBehaviour {

    public float offset = 0.0f;
    public float speed = 0.5f;

    private void Update()
    {
        Vector3 difference = Camera.main.ScreenToWorldPoint(Input.mousePosition) - transform.position; // указывает на эту строку
        difference.Normalize();
        float rotation_z = Mathf.Atan2(difference.y, difference.x) * Mathf.Rad2Deg;
        transform.rotation = Quaternion.Euler(0f, 0f, rotation_z + offset);

        float moveUp = Input.GetAxis("Vertical");
        float moveLeft = Input.GetAxis("Horizontal");
        GetComponent<Rigidbody2D>().velocity = new Vector2(moveLeft, moveUp) * speed;
    }
}

Throws NullReferenceException: Object reference not set to an instance of an object
A bit about the code: in general, I get the cursor coordinates and rotate the character in this direction, but the control keys should remain constant
Help!!!

Answer 1

1) NalRef - execution error, not compilation
2) The error itself must contain a line. Which?
3) GetComponent in Update is a terrible evil. By the way, most likely there is an error in this line - is there Rigidbody2d on the object?

Answer 2

GetComponent only in Start as GavriKos already noted .
NullReferenceException: Object reference not set to an instance of an object - something can be found.