window: Fix CRITICAL on sink finalization
Fixes the following critical error that sometimes happens upon finalization of sink:
(gnome-network-displays:107398): GLib-GObject-WARNING **: 00:19:14.912: invalid unclassed pointer in cast to 'GObject'
(gnome-network-displays:107398): GLib-GObject-CRITICAL **: 00:19:14.912: g_object_set_data: assertion 'G_IS_OBJECT (object)' failed
Fixes: #231 (closed)