Mix between external classes and timeline ActionScript

programming flash
1

I have external document class to instantiate movieclip to the stage which is contained scrollbar. Within this movieclip there’s scrollbar AS ( from Lee Brimelow ) as follows :


import caurina.transitions.*;

var yOffset:Number;
var yMin:Number = 0;
var yMax:Number;


function ScrollBar():void
		{
			sb.thumb.buttonMode = true;
			yMin = 0;
			yMax = sb.track.height - sb.thumb.height;
			sb.thumb.addEventListener(MouseEvent.MOUSE_DOWN, thumbDown);
			stage.addEventListener(MouseEvent.MOUSE_UP, thumbUp);
			
		}


function thumbDown(e:MouseEvent):void
{
	stage.addEventListener(MouseEvent.MOUSE_MOVE, thumbMove);
	yOffset = mouseY - sb.thumb.y;
}

function thumbUp(e:MouseEvent):void
{
	stage.removeEventListener(MouseEvent.MOUSE_MOVE, thumbMove);
}

function thumbMove(e:MouseEvent):void
{
	sb.thumb.y = mouseY - yOffset;
	if(sb.thumb.y <= yMin)
		sb.thumb.y = yMin;
	if(sb.thumb.y >= yMax)
		sb.thumb.y = yMax;
	var sp:Number = sb.thumb.y / yMax;
	Tweener.addTween(body_txt, {y:(-sp*(body_txt.height-masker.height)), time:1});
	e.updateAfterEvent();
}

ScrollBar();

Somehow I got this error message


TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at Body_mc/ScrollBar()
	at Body_mc/frame1()

Can someone show me where did I go wrong?
Thanks a lot