Untitled

AS3 TypeError: Error #1009
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at fl.transitions::Tween/setPosition()
at fl.transitions::Tween/set position()
at fl.transitions::Tween()
at Portfolio_fla::MainTimeline/about_btnOut()

import flash.events.MouseEvent;
import fl.transitions.Tween;
import fl.transitions.easing.*;
import fl.transitions.TweenEvent;

about_btn.buttonMode = true;
about_btn.mouseChildren = false;
about_btn.alpha = 0.3;
about_btn.addEventListener(MouseEvent.MOUSE_OVER, about_btnOver);
function about_btnOver(event:MouseEvent):void
{
    var AboutAlphaOver:Tween = new Tween(about_btn,"alpha",Regular.easeIn,0.3,1,0.1,true);
}
about_btn.addEventListener(MouseEvent.MOUSE_OUT, about_btnOut);
function about_btnOut(event:MouseEvent):void
{
    var AboutAlphaOut:Tween = new Tween(about_btn,"alpha",Regular.easeIn,1,0.3,0.1,true);
}
about_btn.addEventListener(MouseEvent.CLICK, about_btnClick);
function about_btnClick(event:MouseEvent):void
{
    gotoAndPlay(1,"About");
}

var AboutAlphaOut:Tween = new Tween(about_btn,"alpha",Regular.easeIn,1,0.3,0.1,true);

var AboutAlphaOut:Tween = new Tween(event.currentTarget,"alpha",Regular.easeIn,1,0.3,0.1,true);