Here is something new I found out. It seems when you quit by pressing the close button on the Postfish window, it spits out these errors:
(postfish:18904): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `Multibar'
(postfish:18904): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `GtkToggleButton'
(postfish:18904): Gtk-CRITICAL **: file gtktogglebutton.c: line 326 (gtk_toggle_button_get_active): assertion `GTK_IS_TOGGLE_BUTTON (toggle_button)' failed
(postfish:18904): GLib-GObject-WARNING **: invalid uninstantiatable type `<invalid>' in cast to `GtkEntry'
(postfish:18904): Gtk-CRITICAL **: file gtkentry.c: line 3699 (gtk_entry_get_text): assertion `GTK_IS_ENTRY (entry)' failed
Trapped signal 11; saving state and exiting!
This signal almost certainly indicates a bug in the Postfish;
If this version of Postfish is newer than a few months old,
please email a detailed report of the crash along with
processor type, OS version, FFTW3 version, and as much
information as possible about what caused the crash. The best
possible report will outline the exact steps needed to
reproduce the error, ensuring that we at Xiph can fix the
bug as quickly as possible.
-- monty@xiph.org, Postfish revision 6781 2004-05-29 11:53:59Z
(postfish:18904): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `Multibar'
(postfish:18904): GLib-GObject-WARNING **: invalid uninstantiatable type `(null)' in cast to `GtkToggleButton'
(postfish:18904): Gtk-CRITICAL **: file gtktogglebutton.c: line 326 (gtk_toggle_button_get_active): assertion `GTK_IS_TOGGLE_BUTTON (toggle_button)' failed
(postfish:18904): GLib-GObject-WARNING **: invalid uninstantiatable type `<invalid>' in cast to `GtkEntry'
(postfish:18904): Gtk-CRITICAL **: file gtkentry.c: line 3699 (gtk_entry_get_text): assertion `GTK_IS_ENTRY (entry)' failed
Segmentation fault
Exit 139
It doesn't do this when you press the 'Quit' button.