[mutter] clutter/backend: Clear stage window pointer after removing weak ref

[Marge Bot <marge-bot src gnome org>]

commit 50607523bca6ecbb97be41bc947f222cd51e9bd6
Author: Jonas Ådahl <jadahl gmail com>
Date:   Thu Jan 14 16:06:07 2021 +0100

    clutter/backend: Clear stage window pointer after removing weak ref
    
    Otherwise when dispose() is called multiple times we spew warnings on
    tear down.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1670>

 clutter/clutter/clutter-backend.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/clutter/clutter/clutter-backend.c b/clutter/clutter/clutter-backend.c
index b67f6c3b8e..47ea4c4e51 100644
--- a/clutter/clutter/clutter-backend.c
+++ b/clutter/clutter/clutter-backend.c
@@ -98,6 +98,7 @@ clutter_backend_dispose (GObject *gobject)
     {
       g_object_remove_weak_pointer (G_OBJECT (backend->stage_window),
                                     (gpointer *) &backend->stage_window);
+      backend->stage_window = NULL;
     }
 
   G_OBJECT_CLASS (clutter_backend_parent_class)->dispose (gobject);