I've created a new layout. If I try to exit the program (File exit, or click the X) FM displays the message "This operation could not be completed because the target is not part of a related table". When I click ok, the program closes. This does not happen on any other layout.

Somewhere in your script you might be setting a field. However, you are not in the proper context when you try to do so.

Given that you rate yourself, "Advanced," I'll assume that you know what I mean by context.