Determining when animation has finished playing

1

I tried googling but nothing came up (atleast not that i found it) so i tried doing it myself.
In my app im embedding a swf file and then use this code to display it:


_animation_swf = new LaserAnimation() as MovieClipLoaderAsset;
	
	_animation_loader = new SWFLoader();
	_animation_loader.load(_animation_swf);
	
	this.addChild(_animation_loader);

I googled and found out MovieClipLoaderAsset has methods to determine current frame playing and total frames. Perfect for me. I wrote this little class:


package components.init_screen.components
{
	import events.AnimationPlayEvent;
	
	import flash.events.Event;
	import flash.events.TimerEvent;
	import flash.utils.Timer;
	
	import mx.core.MovieClipLoaderAsset;
	
	public class Animation extends MovieClipLoaderAsset
	{
		
		private var _timer:Timer;
		
		public function Animation()
		{
			super();
			
			_timer = new Timer(100);
			_timer.addEventListener(TimerEvent.TIMER, detectPlayback);
			
			this.addEventListener(Event.COMPLETE, startDetection);
			
		}
		
		/**
		 * starts when asset finishes loading
		 * */
		private function startDetection(event:Event):void{
			_timer.start();
		}
		
		/**
		 * is being repeated periodically and detects if animation has finished playing
		 * */
		private function detectPlayback():void{
			if(currentFrame >= totalFrames){
				_timer.stop();
				dispatchEvent(new AnimationPlayEvent(AnimationPlayEvent.PLAY_COMPLETE));
			}
		}


	}
}

and changed the display code to this:


_animation_swf = new LaserAnimation() as Animation;
	
	_animation_swf.addEventListener(AnimationPlayEvent.PLAY_COMPLETE, addLanguageMenu);
	
	_animation_loader = new SWFLoader();
	_animation_loader.load(_animation_swf);
	
	this.addChild(_animation_loader);

The problem is it reports a runtime error: Cannot access a property or method of a null object reference. on line where i add an event listener. If I try commenting that line it seems the app just freezes… no http service response, no animation showing…
Does anyone know what i did wrong?