https://bugs.gentoo.org/965487
Casting to GObject is valid, because GtkWidget embeds GObject.
--- a/goffice-bits/goffice/gtk/go-combo-box.c
+++ b/goffice-bits/goffice/gtk/go-combo-box.c
@@ -173,7 +173,7 @@
 	gdk_device_ungrab (gtk_get_current_event_device (),
 	                   GDK_CURRENT_TIME);
 
-	pdc = g_object_ref (combo_box->priv->popdown_container);
+	pdc = G_OBJECT (g_object_ref (combo_box->priv->popdown_container));
 	g_signal_emit (combo_box,
 		       go_combo_box_signals [POP_DOWN_DONE], 0,
 		       pdc, &popup_info_destroyed);