ActionScript 3.0 :: Button To Move From One To Next Scene

I'm new (like only a couple of weeks here) to Action Scripting and I'm trying to create a button that moves a user from one scene to the next. In doing research this is the script I found that so far has provided me the least amount of errors:

stop();
function nextscene(event:MouseEvent):void {nextScene();
}function prevscene(event:MouseEvent):void {prevScene();
}nextbtn.addEventListener(MouseEvent.CLICK, nextscene);
prevbtn.addEventListener(MouseEvent.CLICK, prevscene);

The one error I DO get is:
Error #1009: cannot access property or method of a null object reference.
at buttontest_fla: main timeline/frame 425()

Attachments: buttontest.zip (4.0 MB)