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

@@ -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;