Trouble with flash components

Thanks for reading!

I have a checkbox in the library of an external swf that is loaded into my main app.

It is all OO, the checkbox is in the library and the import is in the class script.

It works absolutely fine.

THEN,

I tried to use a UIScrollbar in the main app, it’s in the library and it’s being imported correctly. Tested on it’s own, it works fine.

PROBLEM:
Since the scrollbar was added to the main application I receive this error message from the IDE.

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at fl.controls::CheckBox/drawLayout()
at fl.controls::LabelButton/draw()
at fl.core::UIComponent/callLaterDispatcher()

I tried putting the checkbox in the library of the main app as well, with the import code at the top of the main document class, but still the same error is thrown.

Any ideas?

Gav