[gtk+] stack: check for widget before calling is_ancestor()

[Christian Hergert <chergert src gnome org>]

commit bb53f893e25cbff9832e2ab6632ed5e306610e2b
Author: Christian Hergert <christian hergert me>
Date:   Sat Feb 14 18:11:54 2015 -0800

    stack: check for widget before calling is_ancestor()
    
    We can hit this path during the destruction case, where widget has
    been released.
    
    This fixes a lot of superfluous error messages on console.

 gtk/gtkstack.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtkstack.c b/gtk/gtkstack.c
index 2125d66..652b856 100644
--- a/gtk/gtkstack.c
+++ b/gtk/gtkstack.c
@@ -1023,6 +1023,7 @@ set_visible_child (GtkStack               *stack,
       focus = gtk_window_get_focus (GTK_WINDOW (toplevel));
       if (focus &&
           priv->visible_child &&
+          priv->visible_child->widget &&
           gtk_widget_is_ancestor (focus, priv->visible_child->widget))
         {
           contains_focus = TRUE;