A Bug description can be found at http://www.kirupa.com/forum/showthread.php?t=281170
I did spend a lot of time on trying to find a solution for that problem:
Assuming you have a dynamic textbox linked to a Strings-Panel-ID in Frame 1 - 5 you will receive a TypeError #1009 if the Player comes to frame 6.
TypeError: Error #1009: Cannot access a property or method of a null object reference.
Workaround: The error does not occur if the textbox is embedded in a movieclip that has only one frame.