Problems with child reference and links

I’m trying to learn ActionScript, but I end up with errors, even though my code seems fine.

Case 1:
If I go to “Show Pictures” first, the animation plays fine. But when I go to “Play Movie” afterwards, I get a “Child object reference cannot be null” because I want to remove a button from stage. Has the reference changed? It seems completly unlogical!

Also, when I go forth and back betweens the two links, the links suddently stops working! I have added eventlisteners in the first frame, and the instance names of the buttons are the same everywhere.

If someone could help me point out the problem, I would be very grateful!

My project: http://www.stalo.org/external/dockdrop/mediaProject.fla

Thanks.