Collision detection: TypeError: Error #1009: Cannot access a property or method of a null object reference - actionscript-3

Question

I'm trying to make a game but I'm stuck at this point:
in my game you are supposed to evade balls who come in from the right and disappear left. When you hit a ball you'll be redirected to another screen.
Here's the error:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Evader/cleanUpObjects()
at Evader/update()
This is my code:
Evader.as (lines with the hyphens are the erroneous lines):
public class Evader extends MovieClip
{
public var penguin:Penguin;
private var objects:Array;
public function Evader()
{
initGame();
Mouse.hide();
objects = new Array();
addEventListener( Event.ENTER_FRAME, update );
//addEventListener( Event.ENTER_FRAME, hitObject );
}
private function initGame():void
{
penguin = new Penguin();
addChild( penguin );
}
private function update( event:Event ):void
{
if( Math.random() < 0.04 )
{
spawnObject();
}
//--------------------->cleanUpObjects();
}
private function cleanUpObjects():void
{
var currentObject:Object1;
for( var i:int = objects.length-1; i >= 0; i-- )
{
currentObject = objects[ i ];
//----------------------------->if( currentObject.x > stage.stageWidth )
{
removeChild( currentObject );
objects.splice( i, 1 );
}
}
}
private function spawnObject():void
{
var object1:Object1 = new Object1(penguin);
addChild( object1 );
objects.push( object1 );
}
/**
*private function hitObject(event:Event)
*{
* if (penguin.hitTestObject(object1))
* {
* Mouse.show();
* trace("geraakt!");
* removeEventListener( Event.ADDED_TO_STAGE, init );
* removeEventListener( Event.ENTER_FRAME, update );
* removeEventListener( Event.ENTER_FRAME, hitObject );
* Project.instance.switchScreen( "vierde" );
* }
}*/
}
}
Object1.as:
public class Object1 extends MovieClip
{
private var speed:Number;
private var penguin;
public function Object1(aPenguin: Penguin)
{
penguin = aPenguin;
addEventListener( Event.ADDED_TO_STAGE, init );
addEventListener( Event.ENTER_FRAME, hitObject );
}
private function init( event:Event ):void
{
removeEventListener( Event.ADDED_TO_STAGE, init );
addEventListener( Event.REMOVED_FROM_STAGE, unInit );
addEventListener( Event.ENTER_FRAME, move );
speed = -(1 + Math.random() * 2);
x = stage.stageWidth;
y = Math.random() * stage.stageHeight;
}
private function unInit( event:Event ):void
{
removeEventListener( Event.REMOVED_FROM_STAGE, unInit );
removeEventListener( Event.ENTER_FRAME, move );
}
private function move( event:Event ):void
{
this.x += speed;
}
private function hitObject(event:Event)
{
if (penguin.hitTestObject(this))
{
Mouse.show();
trace("geraakt!");
removeEventListener( Event.ADDED_TO_STAGE, init );
removeEventListener( Event.ENTER_FRAME, hitObject );
Project.instance.switchScreen( "vierde" );
}
}
}
}
The thing is, I need to have the hitTestObject function work for all the balls, so that it checks for collisions with all the appearing balls. Penguin is my character who is supposed to evade the balls.
If I comment the hitObject code in Object1.as and uncomment it at Evader.as, I don't know what to put in if (penguin.hitTestObject(???))
Using Objects doesn't work because it will give error #1067: Implicit coercion of a value of type Array to an unrelated type Function.
Could someone help me in getting it work?
Thanks in advance!

Answer 1

Evader is a DisplayObject, specifically type MovielCip. Each display object has a stage field, but the stage field is null until the display object (Evader) gets added to the stage display list (using addChild, eg. stage.addChild( myEvaderInstance) ).
So just from looking at the code you provided, it's possible you haven't added Evader to the display list, therefore its stage is null in the line:
if( currentObject.x > stage.stageWidth )
In which case you wouldn't want to add the event frame listener in its constructor. You'd want to add that listener after it's been added to the stage. So, here's how you can do that. In the Evador constructor:
public function Evader()
{
initGame();
Mouse.hide();
objects = new Array();
//don't update yet, we haven't been added to the display list
//addEventListener( Event.ENTER_FRAME, update );
addEventListener( Event.ADDED_TO_STAGE, onAddedToStage );
}
private function onAddedToStage( e:Event ):void {
removeEventListener( Event.ADDED_TO_STAGE, onAddedToStage );
//add the update listener!
addEventListener( Event.ENTER_FRAME, update );
}
That should do it. We wait till Evader has been added to the display list, then add the event frame listener.