Scroll movie clip left and right

Ok well I got it all running nicely now. The slider fits like it should and instead of using to movies I created a movie clip for the intro animation and the from that loaded the slider into my proLdr.

I just have one last queastion. how can I get rid of this error?

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

it oviously has issues with unloading a null object (what ever that is) from that movie.

stage.addEventListener(Event.RESIZE, resizeHandler);
stage.dispatchEvent(new Event(Event.RESIZE));

function resizeHandler(event:Event=null):void {
    var sw:Number = stage.stageWidth;
    var sh:Number = stage.stageHeight;
    paramF(mc,0,0,sw,sw-mc.width);
}

resizeHandler(null);     // I think this is what it needs to unload.
var tl:MovieClip=this;

paramF(mc,0,0,stage.stageWidth,stage.stageWidth-mc.width);

tl.addEventListener(Event.ENTER_FRAME,scrollF);

function scrollF(e:Event){

mc.x=mc.m*tl.parent.parent.mouseX+mc.b;

}

function paramF(mc:MovieClip,x1:Number,y1:Number,x2:Number,y2:Number){

mc.m=(y1-y2)/(x1-x2);

mc.b=y1-mc.m*x1;

}

I am unloading the movie using:

aboutBtn.addEventListener(MouseEvent.CLICK, aboutLoad);
function aboutLoad(e:MouseEvent):void {
    ldr.unload();
    ldr.source = "about.swf";
}

what causes the movieclip to scroll?  mouse position?  button clicks?  something else?