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

Hello there,

I am using ActionScript 3.0 and I am very new to this. I have created a Scene which has 2 videos with video slider. Along with this there are buttons that lead to another “frame” where I placed a FLVPlayback and referenced the video. All of the buttons connect perfectly from the first frame, but once at the new frame the other buttons do not work anymore.

This is the error that I get:

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

There are no duplicates of the same buttons in any other frames.
The Actionscript layer goes across the whole timeline.

Here is a shot of my timeline: