HELP: GLib-GObject-WARNING **: instance of invalid non-instantiatable type `(null)'

[Guenther Meyer <d s e sordidmusic com>]

hi,

I'm working on a gtk-project for some month; I already solved a lot of 
problems by myself, but I'm still learning.
But now I have a problem, where I can't find any solution:

In my application there's is one window, when I'm closing it, I always can see 
the following messages:

GLib-GObject-WARNING **: instance of invalid non-instantiatable type `(null)'

GLib-GObject-CRITICAL **: g_signal_emit_valist: assertion 
`G_TYPE_CHECK_INSTANCE (instance)' failed

and sometimes the application segfaults after this.

The interesting thing is, that there are other windows, that use exactly the 
same code, but they work without any problems...

any hints or tips?