muxer: force muxer dispoal as part of widget dispose

We need to cleanup state here immediately so that we do not potentially
access the g_class private data after it been finalized. This ensures that
the borrowed reference is dropped by the muxer.

gtk/gtkactionmuxer.c

@@ -858,6 +858,8 @@ gtk_action_muxer_dispose (GObject *object)
 
   g_hash_table_remove_all (muxer->observed_actions);
 
+  muxer->widget = NULL;
+
   G_OBJECT_CLASS (gtk_action_muxer_parent_class)
     ->dispose (object);
 }

gtk/gtkwidget.c

@@ -7357,6 +7357,11 @@ gtk_widget_dispose (GObject *object)
   GtkWidget *widget = GTK_WIDGET (object);
   GtkWidgetPrivate *priv = gtk_widget_get_instance_private (widget);
   GSList *sizegroups;
+  GtkActionMuxer *muxer;
+
+  muxer = g_object_get_qdata (G_OBJECT (widget), quark_action_muxer);
+  if (muxer != NULL)
+    g_object_run_dispose (G_OBJECT (muxer));
 
   if (priv->children_observer)
     gtk_list_list_model_clear (priv->children_observer);