NullReferenceException: Object reference not set to an instance of an object

Hello, I am having problems creating a point system for my 2D space shooter game (I am following the unity tutorial) The problem I am having, is that my game stops when I receive the "points" And then shows me this error: NullReferenceException: Object reference not set to an instance of an object DestroyByContact.OnTriggerEnter2D (UnityEngine.Collider2D other) (at Assets/Scripts/DestroyByContact.cs:30) "

These are my 2 scripts, which should work together:

 using UnityEngine;
 using System.Collections;
 
 public class DestroyByContact : MonoBehaviour {
 
     public GameObject explosion;
     public GameObject playerExplosion;
     public int scoreValue;
     private GameController gameController;
 
     void Start()
     {
         GameObject gameControllerObject = GameObject.FindWithTag ("GameController");
         if(gameControllerObject != null)
         {
             gameController = gameControllerObject.GetComponent <GameController>();
         }
         if(gameController = null)
         {
             Debug.Log ("Cannot find Gamecontroller Script");
         }
     }
 
     void OnTriggerEnter2D(Collider2D other) {
         Instantiate(explosion, transform.position, transform.rotation);
 
         if(other.tag == "Player"){
             Instantiate(playerExplosion, other.transform.position, other.transform.rotation);
         }
         gameController.AddScore (scoreValue);
         Destroy(other.gameObject);
         Destroy(gameObject);
     }
     
 
 }
 


 using UnityEngine;
 using System.Collections;
 
 public class GameController : MonoBehaviour {
 
     public GameObject hazard;
     public Vector2 spawnValues;
     public int hazardCount;
     public float spawnWait;
     public float startWait;
     public float waveWait;
 
     public GUIText scoreText;
     private int score;
 
     void Start () {
         score = 0;
         UpdateScore();
         StartCoroutine (SpawnWaves ());
     }
 
 
     IEnumerator SpawnWaves () {
     
         yield return new WaitForSeconds (startWait);
         while(true){
             for (int i = 0; i < hazardCount; i++){
                 Vector2 spawnPosition = new Vector2(Random.Range(-spawnValues.x, spawnValues.x), spawnValues.y);
                 Quaternion spawnRotation = Quaternion.identity;
                 Instantiate(hazard, spawnPosition, spawnRotation);
                 yield return new WaitForSeconds (spawnWait);
             }
             yield return new WaitForSeconds(waveWait);
         }
     }
 
     public void AddScore(int newScoreValue)
     {
         score += newScoreValue;
         UpdateScore();
     }
 
     void UpdateScore()
     {
         scoreText.text = "Score: " + score;
     }
 
 }

Any help is appreciated, thanks!