Document class must be on first frame!? .. Error #1009

Hey, I have a project starting at frame one and I have a document class attached to my project. I found a working preloader thing to have but when I move the project one frame ahead in the main timeline suddenly the document class doesn’t work anymore :confused:

This is what the “output”-properties window says:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at classes::BackForwardClass()

appreciate it!