I suspect that the XAML in which RadDataFilter is defined is not parsed until the control becomes visible and that is why its Source is not set. Or something similar.
Anyway, can you please send me a very very small dummy project that reproduces this behavior and will immediately debug in order to see what is actually going on. Once I know the reason, I will be able to suggest the best approach.
Thanks in advance.
All the best,
the Telerik team
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