NullReferenceException: Object reference not set to an instance of an object; Pasek zycia poruszajacy sie za graczem

Question

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

public class HealthBar : MonoBehaviour {

    float x;
    float y;
    float dx;
    float dy;


	// Use this for initialization
	void Start ()
    {

        dx = transform.position.x - Camera.main.gameObject.transform.position.x;
        dy = transform.position.y - Camera.main.gameObject.transform.position.y;
	}
	
	// Update is called once per frame
	void Update (){

        Pozycja();

	}
    void Pozycja()
    {

        x = Camera.main.gameObject.transform.position.x + dx;
        y = Camera.main.gameObject.transform.position.y + dy;
        transform.position = new Vector3(x, y, transform.position.z);

    }
}