"NullReferenceException" error - nullreferenceexception

Question

I'm trying to add a scoring system to my game. However I get this error whenever I play and the asteroids wont destroy anymore when they collide with either the player or a bullet.
My error message is this:
NullReferenceException: Object reference not set to an instance of an object
DestroyByContact.OnTriggerEnter2D (UnityEngine.Collider2D other) (at Assets/Scripts/DestroyByContact.cs:47)
I should note that all game objects have the correct tags on them as well.
And some Code:
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){
if (other.tag == "Boundary") {
return;
}
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;
using UnityEngine.UI;
public class GameController : MonoBehaviour {
public GameObject[] asteroids;
public Vector3 spawnValues;
public int asteroidCount;
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 (asteroidCount > 0) {
for (int i = 0; i < asteroidCount; i++) {
GameObject asteroid = asteroids[Random.Range(0, asteroids.Length)];
Vector3 spawnPosition = new Vector3 (spawnValues.x, Random.Range (-spawnValues.y, spawnValues.y), spawnValues.z);
Quaternion spawnRotation = Quaternion.identity;
Instantiate (asteroid, spawnPosition, spawnRotation);
yield return new WaitForSeconds (spawnWait);
}
yield return new WaitForSeconds (waveWait);
if (asteroidCount <= 95) {
asteroidCount += 5;
}
}
}
public void AddScore (int newScoreValue) {
score += newScoreValue;
UpdateScore ();
}
void UpdateScore () {
scoreText.text = "Score:" + score;
}
}

Answer 1

I'd say that better idea would be to set gameController after Instantiate inside Controller. If I understand it right, your GameController spawns asteroids that have DestroyOnContact attached from prefab/predefined collection?
If so, consider doing that way:
GameController
var go = Instantiate (asteroid, spawnPosition, spawnRotation) as GameObject;
if (destroyable != null)
{
var destroyable = go.GetComponent<DestroyByContact>();
if (destroyable == null)
{
//Something else got instantiated, destroy it as we don't need it here
Destroy(destroyable);
}
else
{
destroyable.gameController = this;
}
}
DestroyOnContact
//just change accessor from private to public
public GameController gameController;
//Get rid of stuff that would set gameController in Start()
//Also check if gameController != null before you call .AddScore()