I'm using CS4 and AS3

I am getting no compile errors in my code, but in the output window, I keep receiving the following:

TypeError: Error #1009: Cannot access a property or method of a null object reference.

This prompt appears before I click on the button that is supposed to run the movie clip, so it appears that it is not the problem.

Can anyone tell me what I am doing wrong? I am working with buttons and movie clips; all of my objects are display objects or display containers, so I have no idea what the reference to a null object is.
I have not created any custom classes for this, and don't think I would have to for such a simple project.

Rick