Issue referencing stage

Question

I think my issue is to do with referencing the stage, but not to sure.  When I try
var screen:Sprite = new Sprite();
screen.graphics.beginFill(0x111111, .75);
screen.graphics.drawRect(0,0, stage.stageWidth, stage.stageHeight);
screen.graphics.endFill();
I get the error
TypeError: Error #1009: Cannot access a property or method of a null object reference.
If I change stage.stageWidth, stage.stageHeight to numbers, it works fine.  So I think the problem is to do with referencing the stage.  Whats confusing me though is if I set the above to numbers, later on in my code I have
TweenLite.to(loaderHolder, 1, { x: (stage.stageWidth - loaderHolder.width) / 4, y: (stage.stageHeight - loaderHolder.height) / 6, onComplete:zoomInFinished, scaleX: 1, scaleY: 1 });
So I am referencing the stage there.  However, this works without a problem.
If it makes a difference, I am working within a movieclip which is in my library, and added to the stage on the main timeline.  Within this movieclip, I do things like stage.addChild, so this works without a problem.  Why might the first bit of code be throwing a null reference error?
cheers

Answer 1

because you're trying to reference the stage before before your object is added to the display list.  to remedy, use the addedtostage event before trying to reference the stage.