as3 - Error #1009, an object is null and bugged with timer - actionscript-3

Question

When the player touches the door, the next level is supposed to be added and the previous level should be removed. However in the game, the next level does get added and everything works, but the output shows this issue.
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at flash.utils::Timer/tick()
This is the enterframe:
if (player.collisionArea.hitTestObject(door0))
{
var timer: Timer = new Timer(1000, 1);
timer.addEventListener(TimerEvent.TIMER_COMPLETE, fade);
timer.reset();
timer.start();
}
This is the timer:
function fade(event: TimerEvent)
{
removeEventListener(TimerEvent.TIMER_COMPLETE, fade);
var pageTwo: PageTwo = new PageTwo;
parent.addChild(pageTwo);
this.parent.removeChild(this);
}
This is in the "previous level" class. "this" is itself (previous level) and "pageTwo" is (next level).
So, the output indicates the error is at "parent.addChild(pageTwo);". However if I remove that, the output indicates the issue is from "this.parent.removeChild(this);
I assume it's because the child is removed that's why there is a null issue. But how do I solve this error coming from the output? Am I removing the movieclip the wrong way?

Answer 1

Your problem is coming from this line :
parent.addChild(pageTwo);
because in your code as this condition
if (player.collisionArea.hitTestObject(door0))
is true, you will create another Timer object which when its TimerEvent.TIMER_COMPLETE event is fired, it will try to add a new PageTwo instance to a null parent and that's why the error is fired.
So to avoid that, you can remove the Event.ENTER_FRAME event listener at the first time when that condition is true or if you still need it (the Event.ENTER_FRAME event listener) you have to verify your condition to avoid such behavior.
Also, your timer var should be declared as global to be accessible from the fade() function to be able to remove its TimerEvent.TIMER_COMPLETE event listener which you are trying to remove from your current object.
Hope that can help.