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

1

Hey guys I’m new to code, and here’s my problem. Any advice??

“TypeError: Error #1009: Cannot access a property or method of a null object reference.
at type_fla::whole_1/thumbUp()”

Here’s my code

stop();

import caurina.transitions.*;

var xOffset:Number;
var xMin:Number = 0;
var xMax:Number = sb.track.width - sb.thumb.width;

sb.thumb.addEventListener(MouseEvent.MOUSE_DOWN, thumbDown);
stage.addEventListener(MouseEvent.MOUSE_UP, thumbUp);

function thumbDown(e:MouseEvent):void
{
stage.addEventListener(MouseEvent.MOUSE_MOVE, thumbMove);
xOffset = mouseX - sb.thumb.x;
}

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

function thumbMove(e:MouseEvent):void
{
sb.thumb.x = mouseX - xOffset;
if(sb.thumb.x <= xMin)
sb.thumb.x = xMin;
if(sb.thumb.x >= xMax)
sb.thumb.x = xMax;
var sp:Number = sb.thumb.x / xMax;
Tweener.addTween(content, {x:(-sp*(content.width-masker.width)),
time:1.5});
e.updateAfterEvent();
}