How are you opening the RadWindow? Can you confirm it is after the Sys.Application.Load event? If you use its client-side API, can you confirm the first call is to the show() method of the RadWindow (or, respectively, radopen() or the manager's open() methods)? The idea is that the popup needs to be created before it can be modified?
Can you reproduce this behavior with our online demos, e.g., this one? I am asking for this because we are not aware of such a problem and at this point the only other thing I can suggest is upgrading to the latest version we have to see if this helps. The Q3 2013 BETA release just wen out, so you can download it and give it a try.
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