avoid assert in DoGetBestSize() for an empty tree

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@43689 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/treebase.cpp

@@ -162,7 +162,9 @@ wxSize wxTreeCtrlBase::DoGetBestSize() const
     else // use precise, if potentially slow, size computation method
     {
         // iterate over all items recursively
-        wxGetBestTreeSize(this, GetRootItem(), size);
+        wxTreeItemId idRoot = GetRootItem();
+        if ( idRoot.IsOk() )
+            wxGetBestTreeSize(this, idRoot, size);
     }
 
     // need some minimal size even for empty tree