Hi,
You could ask this issue in telerik forum. Since RadControls are third party Siverlight products, I would like to suggest you reference the following address:
RadWindows for Silverlight
http://www.telerik.com/help/silverlight/radwindow-overview.html
Telerik Community & Support
http://www.telerik.com/community/forums/silverlight.aspx
Best Regards,
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