don't call EnsureVisible(tree->GetRootItem()) for a tree with hidden root, this crashes under MSW

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@40855 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

samples/widgets/widgets.cpp

@@ -578,7 +578,9 @@ void WidgetsFrame::InitBook()
 
     // but ensure that the top of the tree is shown nevertheless
     wxTreeCtrl * const tree = m_book->GetTreeCtrl();
-    tree->EnsureVisible(tree->GetRootItem());
+
+    wxTreeItemIdValue cookie;
+    tree->EnsureVisible(tree->GetFirstChild(tree->GetRootItem(), cookie));
 #endif // USE_TREEBOOK
 }