Segmentation fault when clicking a GtkWindow

[Hubert Mauchle <hmauchle magnet ch>]

From a top level window I open a new, modal top level window
with some widgets in it. If I close the modal window using
the "x" in the top bar and re-open the modal window, I get
an empty window (without the defined widgets).
If I then click either the new window or the main window,
Gtk3 crashes with Segmentation fault:

gtk_widget_get_window: assertion 'GTK_IS_WIDGET (widget)' failed
Segmentation fault

I use Debian Linux Testing and my software stack has been updated
recently.

The MWE is attached as mwe.pl, the corresponding glade file is 
glade_versuch04.glade.

What am I missing? Any pointers are appreciated.

Regards,
Hubert

Attachment: mwe.pl
Description: Perl program

Attachment: glade_versuch04.glade
Description: application/glade