Please accept our apology for the delayed answer on this question. We tested a simple scenario using the RadTimePicker control in new project created in Blend and it seems that the code works as expected. Our set up is Windows 10 PC + Blend 2015 and our Latest official release of the UWP suite of controls (manual installation).
However, it your setup is the same most probably the issue is caused by other factors. Currently we are not aware for any reasons that may cause such exception. This is why we would ask you to provide us with additional details about your application. Any details that can help us recreate your exact scenario on our side are welcome (e.g. code in pages, pictures, descriptions)
Pavel R. Pavlov
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