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Similar problem here. It only happens if there is at least
one file open in the editor when I quit the application.
Whichever method is used to quit (I mean X button or menu)
gives the same error messages:

Gtk-CRITICAL **: file gtkwidget.c: line 4870
(gtk_widget_ref): assertion `widget != NULL' failed.

Gtk-CRITICAL **: file gtkwidget.c: line 1691
(gtk_widget_unrealize): assertion `widget != NULL' failed.

Gtk-CRITICAL **: file gtkwidget.c: line 1250
(gtk_widget_unparent): assertion `widget != NULL' failed.
Erreur de segmentation

So the problem seems to lie in the code path used to save
the current state of the open windows/files when quitting.

For your knowledge, I'm using:
- Debian GNU/Linux "testing" updated (almost) daily
- KDE 3.3.2
- wxPlatform __WXGTK__ 2.4.2.4 (as reported by BC
"about" window)
- Python 2.3.4 (as reported by BC "about" window)
- Boa Constructor 0.3.0
All software are installed from regular Debian packages
and run on an x86 box (AMD ThunderBird on Asus mobo).