This message has nothing to do with the VirtualizingWrapPanel. It is caused because you have added RadWindow in XAML and it is not a RootControl.
We remade big part of RadWindow for Q2 2011 and decided that this is the correct way to do things. Previously when RadWindow was declared in XAML we were removing it from its Parent and put it in a popup then show it. Clients tend to bind elements in the window to another XAML element and this binding getting broken and there were also other problems like panels layout being refreshed because of removing the window etc...
We believe that it is better practice to either create RadWindow in code behind or use it as a root element of a UserControl especially in WPF as there it is supposed that new windows are always root elements. So that is the reason that we changed the way RadWindow works when declared in XAML.
So the only thing you have to do is to put the RadWindow in a new UserControl, more info on how to do this you can find in this help thread
the Telerik team
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