void IOutlinedDocument.ReleaseOutlineWidget()
        {
            if (outlineTreeView == null)
            {
                return;
            }

            Gtk.ScrolledWindow w = (Gtk.ScrolledWindow)outlineTreeView.Parent;
            w.Destroy();
            outlineTreeView.Destroy();
            outlineTreeStore.Dispose();
            outlineTreeStore = null;
            outlineTreeView  = null;
        }