Movie Clip with Close button won't stop looping

Hi EE

I have a Flash CS5.5 file with 1 frame in the timeline. In that frame I have a button which when clicked opens a movie clip. The movie clip is a 24-frame animation (it's a motion tween), and has a Close button on the Frame 24. However, when I test the FLA, the movie clip loops and I get the error:

TypeError: Error #1010: A term is undefined and has no properties.
      at scrCustomerMaint_fla::MainTimeline/frame1()

My code is:

modAnimate.visible = false;
btnCustCode.addEventListener(MouseEvent.CLICK, showPopup);
function showPopup(event:MouseEvent):void
{
	modAnimate.visible = true;
}

modAnimate.btnClose.addEventListener(MouseEvent.CLICK, hidePopup);
function hidePopup(event:MouseEvent):void
{
	modAnimate.visible = false;
}

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Debug says the problem is on the "modAnimate.btnClose" line. I guess it's because btnClose isn't available at that point, but I don't know how to fix the problem. I've tried adding a stop() function to Frame 24 of the movie clip but that just throws up more errors. Can anyone please help?

Incidentally, if I use a similar movie clip - one that has no motion tween - the above scenario works perfectly, i.e. main timeline button opens the movie clip, and the movie clip's Close button closes the movie clip.

TIA