Hey... I'm having some trouble wit a tween in my movie...

What I want the tween to do is that when it's finished, repeat the loop in the opposite direction.

As far as I can see, this should work, although when I try it I get this error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at AnotherMazeGame_fla::MainTimeline/Tween8aHandler()
at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at fl.transitions::Tween/set time()
at fl.transitions::Tween/nextFrame()
at fl.transitions::Tween/onEnterFrame()

Can anyone help?

Code:
var tween8a:Tween=new Tween(motion8a_mov, "x", Regular.easeInOut, 67.3, 480.3, 168, false);
var tween8b:Tween=new Tween(motion8b_mov, "x", Regular.easeInOut, 480.3, 67.3, 168, false);

tween8a.addEventListener(fl.transitions.TweenEvent.MOTION_FINISH, Tween8aHandler);
tween8b.addEventListener(fl.transitions.TweenEvent.MOTION_FINISH, Tween8bHandler);

function Tween8aHandler(e:fl.transitions.TweenEvent):void{
	if(motion8a_mov.x<=67.3){
		tween8a.start();
	}
	else if(motion8a_mov.x>=480.3){
		tween8a.yoyo();
	}
}
function Tween8bHandler(e:fl.transitions.TweenEvent):void{
	if(motion8b_mov.x>=480.3){
		tween8b.start();
	}
	else if(motion8b_mov.x<=67.3){
		tween8b.yoyo();
	}
}