backport changes for D Mars

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_4_BRANCH@22857 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/dirctrlg.cpp

@@ -746,7 +746,7 @@ void wxGenericDirCtrl::OnExpandItem(wxTreeEvent &event)
 
     // VS: this is needed because the event handler is called from wxTreeCtrl
     //     ctor when wxTR_HIDE_ROOT was specified
-    if (m_rootId == 0)
+    if (!m_rootId.IsOk())
         m_rootId = m_treeCtrl->GetRootItem();
 
     ExpandDir(parentId);