Исключение NullReferenceException: как исправить код?

using UnityEngine;
using UnityEngine.UI;
using System.Collections;
 
public class MovingPlayer : MonoBehaviour 
{
    public float speed;
    private Rigidbody rb;
 
    private int score;
    public Text scoreText;
 
    public GameObject active;
 
    private door DK;
    public GameObject door;
    
    void Start ()
    {
        SetScoreText ();
        score = 0;
        rb = GetComponent<Rigidbody>();
    }
    
    void FixedUpdate ()
    {
        if (score == 10) 
        {
            active.SetActive (true);
        }
 
        float moveHorizontal = Input.GetAxis ("Horizontal");
        float moveVertical = Input.GetAxis ("Vertical");
        
        Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
        
        rb.AddForce (movement * speed);
    }
 
    void OnTriggerEnter(Collider other)
    {
        if (other.CompareTag ("PickUp")) 
        {
            score=score+1;
            SetScoreText ();
            Destroy(GameObject.Find("Star"));
        }
 
        if (other.CompareTag("Door")&& DK.key==true)
        {
            score=score+5;
            SetScoreText ();
        }
    }
     public void SetScoreText()
    {
        scoreText.text = "Score:" + score.ToString ();
    }
}

using UnityEngine;
using System.Collections;
 
public class Key : MonoBehaviour 
{
    private door DK;
 
    void Start()
    {
        DK = GameObject.FindGameObjectWithTag ("Door").GetComponent<door>();
    }
    
    void OnTriggerEnter(Collider col)
    {
        DK.key = true;
        Destroy (gameObject);
    }
}

DK = GameObject.FindGameObjectWithTag ("Door").GetComponent<door>();

private door DK;
public GameObject door;
    void Start()
    {
        DK = door.GetComponent<door>();
    }