//this is the event that fires every frame for thumbContainer's
function scrollThumbs(e:Event):void {
	//we initialize the thumbContainer variable from the event data passed to this function
	var thumbContainer:Sprite = e.target as Sprite;
	if (thumbContainer.width>0)
	{
	
	//we scroll the thumbContainer by the amount specified at the top
	thumbContainer.x -= scrollAmount;
	trace(thumbContainer.width);  // ALWAYS GIVES 510
	//if the thumbContainer has scrolled past the mask, we no longer need it, so remove it and add a new group of thumbnails to the end of the main container
	if(thumbContainer.x <= -(thumbContainer.width)){
		thumbContainer.parent.removeChild(thumbContainer);
		createThumbs();
	}	
}
}


OUTPUT 
-------

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TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at Untitled_fla::MainTimeline/scrollThumbs()
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TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at Untitled_fla::MainTimeline/scrollThumbs()
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TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at Untitled_fla::MainTimeline/scrollThumbs()
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