Actually these clarifications helped a lot. I have managed to reproduce the exception. As Silverlight 4 does not support binding to DynamicObject, we have implemented our own mechanism that mimics the default TwoWay binding behavior, in order to tackle this framework limitation. Unfortunately, it is impossible for us to guess the custom logic behind every DataTemplate that might be set as EditorTemplate, so RadPropertyGrid does not supports custom DataTemplates when it is bound to a DynamicObject instance. We have some ideas for introducing a solution to this problem, but for now you should rely only on the default editors. However, we will fix the exception with our next internal build, which will be available in the beginning of the next week.
the Telerik team
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