Steps to reproduce:
1. Run the test_pygtk_crash.py test file with filechooser.glade in the same directory.

Stack trace:
TypeError: Cannot create a consistent method resolution
order (MRO) for bases ImplementorIface, CellEditable, Editable, Buildable
**
ERROR:/build/buildd/pygobject-2.15.3/gobject/pygobject.c:920:pygobject_new_full: assertion failed: (tp != NULL)
Aborted (core dumped)

Other information:
As a developer of LottaNZB (a PyGTK app), I recently gave our application a try on Ubuntu Intrepid, with the latest updates applied (python-gtk2 2.13, python-gobject 2.15.3). The application unexpectedly crashed when opening the file selection dialog with the following error message:

TypeError: Cannot create a consistent method resolution
order (MRO) for bases ImplementorIface, CellEditable, Editable, Buildable
**
ERROR:/build/buildd/pygobject-2.15.3/gobject/pygobject.c:920:pygobject_new_full: assertion failed: (tp != NULL)
Aborted (core dumped)

Trying to locate the problem, I dove into the code of Kiwi, the PyGTK framework we use in our application. I finally found the line of code which caused this issue. To my surprise it wasn't a Kiwi-specific problem, as I was able to reproduce it with pure PyGTK code. This crash happens whenever the developer tries to access the property 'focus_widget' of a gtk.FileChooserDialog or a dialog that contains a gtk.FileChooserWidget after the dialog's show() method was invoked.

This issue can only be reproduced using Intrepid, but not using Hardy, that's why I assume that the bug is caused by certain changes recently made in python-gtk or python-gobject

The only way to prevent the application from crashing is to ensure that this property isn't accessed (when using Kiwi by overwriting the check_focus method).

Test case is attached.