According to
http://docs.telerik.com/devtools/wpf/api/html/e_telerik_windows_controls_radpropertygrid_preparededitor.htm , and also to the auto completed event handler that is being created with double Tab, this should have worked, but when I try that, I get the exception above.
It looks like it is actually expecting RadRoutedEventHandler (looking at the cast inside the code), but trying to create a function with event args of type RadRoutedEventArgs generage (as expected) a compilation error.
Is this event even useful for anything?