The night light dialog is both marked as "destroy_with_parent" and explicitly
destroyed in the panel. Drop one of these.

Causes the warning after opening the dialog then closing the app:
(gnome-control-center:19887): Gtk-CRITICAL **: 11:00:01.370: gtk_widget_destroy: assertion 'GTK_IS_WIDGET (widget)' failed

This bug was introduced in ed36688c