View-model: fix failed assertions on dispose
When switching from grid view to list view the failing assertions are printed
(org.gnome.NautilusDevel:2): GLib-GObject-CRITICAL **: 06:46:36.855: g_signal_handlers_disconnect_matched: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed
(org.gnome.NautilusDevel:2): GLib-GObject-CRITICAL **: 06:46:36.855: g_object_unref: assertion 'G_IS_OBJECT (object)' failed
!817 (merged) changes the internal_model from a GListStore to GtkSortListModel, but same issue exists.