GladeWidget: unset parent and object on rebuild

We need to make sure internal widgets parent and object are unset
while rebuilding to avoid having a GladeWidget with a dead object.

gladeui/glade-widget.c

@@ -1805,6 +1805,9 @@ glade_widget_extract_children (GladeWidget *gwidget)
 
           extract_list = g_list_prepend (extract_list, extract);
 
+          /* Internal GladeWidget are no longer needed */
+          glade_widget_set_object (gchild, NULL);
+          glade_widget_set_parent (gchild, NULL);
         }
       else if (gchild || GLADE_IS_PLACEHOLDER (child))
         {