TypeError: Error #1009: Cannot access a property or method of a null object reference

1

Hi,

Anyone got an ideas why I get this following error message please.

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Particle/update()

Code as below.

package {
import flash.display.MovieClip;
import flash.events.Event;

public class Particle extends MovieClip {
	var xVel:Number;
	var yVel:Number;
	

	function Particle() {
		xVel=Math.random()*20-10;
		yVel=Math.random()*20-10;
		this.addEventListener(Event.ENTER_FRAME, update);
	}

	private function update(e:Event):void {
		
		this.y+=yVel;
		this.x+=xVel;
		
		
		if (this.x > stage.stageWidth)
		{
			removeEventListener(Event.ENTER_FRAME, update);
			parent.removeChild(this);
		}
		
		if (this.y > stage.stageHeight)
		{
			removeEventListener(Event.ENTER_FRAME, update);
			parent.removeChild(this);
		}
		
		
	}
}

}