Please can somebody help me with this. I have been trying the ENTIRE day and just can't figure out what is going on.

I have a movieclip on the first frame with a button on the end that says enter. This will be my intro basically.
The enter button works and goes where it is meant to go but I just keep getting this error in the output:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at site_fla::MainTimeline/frame2()
at flash.display::Sprite/constructChildren()
at flash.display::Sprite()
at flash.display::MovieClip()
at site_fla::navBar_mc_4()
at flash.display::MovieClip/gotoAndStop()
at site_fla::intro_mc_1/enterClicked()

All the other buttons seem to work and it is the same as those ones....WHY?????
please help me.....

forum.fla