NullReferenceException: Object reference not set to an instance of an object script_bullet.OnTriggerEnter (UnityEngine.Collider other) (at Assets/script_bullet.js:35)

Questions & Answers
1

I am getting an Null Reference Exception when trying to call CreateAsteroid() (which is inside my script_asteroid file) from within script_bullet. I am not exactly sure how I should be calling the asteroid. Can somebody show me what I am doing wrong? Thanks in advance.

//Bullet Script
#pragma strict

//Inspector Variables

var bulletSpeed 		: float = 40.0;				//Bullet Speed
var bulletBounds		: float = 15;				//Bullet Boundry 
var asteroid 			: script_asteroid;
asteroid 				= GetComponent(script_asteroid);
var explosion			: Transform;


function Start () {

}

function Update () {
	var moveBullet		: float = bulletSpeed * Time.deltaTime;
	
	//Moves Player based on stored input data
	transform.Translate(moveBullet, 0,0);
	
	if(transform.position.x >= bulletBounds)
		Destroy(gameObject);
}

function OnTriggerEnter (other : Collider) 
{
	//Print what the bullet is colliding with
	print("bullet collison: " + other.gameObject.tag);
	
	//Check for collision with asteroid
	if(other.gameObject.tag == "Asteroid")
	{
		asteroid.CreateAsteroid();
		
		//Create the explosion on impact
		Instantiate(explosion,transform.position,transform.rotation);
		Destroy(other.gameObject);
		Destroy(gameObject);
	}
	
}
//Player Script
#pragma strict

//Inspector Variables

var asteroidSpeedHMin 		: float = 1.0;				//Asteroid Horizontal Speed Min
var asteroidSpeedHMax 		: float = 6.0;				//Asteroid Horizontal Speed Max
var asteroidSpeedV 			: float = 10.0;				//Asteroid Vertical Speed
var asteroidBoundsH			: float = 15;				//Limits for Asteroid movement Horizontal
var asteroidBoundsV			: float = 10;				//Limits for Asteroid movement Vertical
var asteroid					: Transform;
var asteroidHealth			: int	= 3;

//Private Variables
private var asteroidPosition;
private var asteroidRotation;
private var transH;
private var transV;

function Start () {
	asteroidPosition = Vector3(transform.position.x,Random.Range(-8,8),0);
	asteroidRotation = transform.rotation;
	transH 			= -Random.Range(asteroidSpeedHMin,asteroidSpeedHMax)*Time.deltaTime;
	transV 			= Random.Range(-asteroidSpeedV,asteroidSpeedV)*Time.deltaTime;
}

function Update () {


	//Moves Asteroid based on Random numbers
	transform.Translate(transH,transV,0);
	
	if(transform.position.x >= asteroidBoundsH || transform.position.x <= -asteroidBoundsH || transform.position.y >= asteroidBoundsV || transform.position.y <= -asteroidBoundsV) {
		CreateAsteroid();
		Destroy(gameObject);
	}
}

function CreateAsteroid() {
	Instantiate (asteroid, asteroidPosition, asteroidRotation);
}
2

Ok so firstly don’t put code outside a function in Unity script - put the

asteroid = GetComponent(script_asteroid);

Inside start.

Next - what is script_asteroid attached to ? Because at the moment it appears it thinks its attached to your bullet - which seems unlikely. The GetComponent is looking at the GameObject it is attached to and trying to get the asteroid component from that - which is the bullet.

It appears CreateAsteroid is actually on the player - so you need to get a reference to that. Easiest way might be a singleton on the player - like this:

//Player Script
#pragma strict

//Inspector Variables

var asteroidSpeedHMin    : float = 1.0;          //Asteroid Horizontal Speed Min
var asteroidSpeedHMax    : float = 6.0;          //Asteroid Horizontal Speed Max
var asteroidSpeedV         : float = 10.0;          //Asteroid Vertical Speed
var asteroidBoundsH        : float = 15;          //Limits for Asteroid movement Horizontal
var asteroidBoundsV        : float = 10;          //Limits for Asteroid movement Vertical
var asteroid              : Transform;
var asteroidHealth       : int  = 3;

//Private Variables
private var asteroidPosition;
private var asteroidRotation;
private var transH;
private var transV;

public static var instance : Player; //Presuming it's called Player!
function Awake()
{
      instance = this;
}

function Start () {
    asteroidPosition = Vector3(transform.position.x,Random.Range(-8,8),0);
    asteroidRotation = transform.rotation;
    transH          = -Random.Range(asteroidSpeedHMin,asteroidSpeedHMax)*Time.deltaTime;
    transV          = Random.Range(-asteroidSpeedV,asteroidSpeedV)*Time.deltaTime;
}

function Update () {


    //Moves Asteroid based on Random numbers
    transform.Translate(transH,transV,0);

    if(transform.position.x >= asteroidBoundsH || transform.position.x <= -asteroidBoundsH || transform.position.y >= asteroidBoundsV || transform.position.y <= -asteroidBoundsV) {
       CreateAsteroid();
       Destroy(gameObject);
    }
}

function CreateAsteroid() {
    Instantiate (asteroid, asteroidPosition, asteroidRotation);
}

The any time you like you can do:

  Player.instance.CreateAsteroid();

We’ve put together some tutorials on this stuff on Unity Gems