Nested call to funtion passing arguments

1

I have a movie clip called “aaa_test_clip.” One the first frame of aaa_test_clip, I created a button called “thebutton.” On the second frame of aaa_test_clip I have a blue graphic.

on the first frame of the root of my movie I wrote this function:


stop();
function doit(testvar:String):void {
    this["aaa_"+testvar+"_clip"].gotoAndStop(2);
}

If I write on the next line

doit("test");

the function works and it takes you to the second frame of aaa_test_clip with the blue graphic.

My problem is that I would like to have the button call this function and the button function defined in the nested movie clip like this:


stop();
thebutton.addEventListener(
  MouseEvent.MOUSE_UP,
  function(evt:MouseEvent):void {
    MovieClip(this.root).doit("test");
  }
);

but when I click thebutton I get an error message:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at MethodInfo-6()

What am I doing wrong here? I attached my fla.