[veisss]

наверное

private function on_MouseDown(event:MouseEvent):void
		   {
			    stage.addEventListener(MouseEvent.MOUSE_MOVE, on_MouseMove);
		   }	
 
 private function on_MouseUp(event:MouseEvent):void
		   {
			   stage.removeEventListener(MouseEvent.MOUSE_MOVE, on_MouseMove);
		   }

stage и есть тот null