We have tried to reproduce the exception, but we were not able to do so. In addition as it happens sometimes we have tried different scenarios involving a lot of clicking, but still no luck.
So I am attaching my test application and it would be great if you can check it out and let me know if I am not missing something and can you observe the issue in there. Also if the issue cannot be reproduced in the attached project it would be helpful if you can send us the application in which you are observing it and if possible with any steps to reproduce.
Anyway it seems from the stack trace like the AnimationManager is trying to animate some missing element. So as a workaround you can disable the AnimationManager as shown in the project on the first RadComboBox:
Hope this would help.
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