Error??

I am getting an error at runtime.....

I check my code before runtime and it checks out!

Here is the error....
http://www.sbdesigns.ca/tester/error.jpg

Here is my code...

Code:

---

root.moveButton_mc.addEventListener(MouseEvent.MOUSE_DOWN, startMove);

root.closeButton_mc.addEventListener(MouseEvent.MOUSE_UP, closeWindow);


function startMove(event:MouseEvent):void
{
        this.stage.nativeWindow.startMove();
}
function closeWindow(event:MouseEvent):void
{
        this.stage.nativeWindow.close();
}

---

It is basically a tutorial video with a close and drag buttons.

What is causing this error, any suggestions?

How did you set up your buttons?