ActionScript 3.0 :: Make Video Stop Video When Leaving A Frame?

I know how to make a video (instanced as showreel) stop when clicking on a button that that navigates to another section/frame, since otherwise I would still listen to that video while not visible. The problem is when I'm in another frame that doesn't have that video, when clicking on the menu to go to whichever other frame, it looks for that it to stop if it's playing and it and doesn't find it, and so there's an error.
 
How can solve this? Maybe I should put an order saying something like if there exists a video in that frame instanced as sowreel, stop it, otherwise don't do anything as there is nothing to be stopped. Or assign the order to the frame (which is called motion), but I don't know how to do that, can someone help? The code I have is: 
 
import fl.video.VideoEvent;
function onClick(evt:MouseEvent):void {    if( showreel.playing ){        showreel.stop();}    gotoAndStop(evt.currentTarget.name);}
motion.addEventListener(MouseEvent.CLICK, onClick);web.addEventListener(MouseEvent.CLICK, onClick);logo.addEventListener(MouseEvent.CLICK, onClick);photo.addEventListener(MouseEvent.CLICK, onClick);