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You may encounter this if in the CompletedEvent handler you try to access a visual element (maybe the RadBusyIndicator?) from the Tab that has been closed. If this is the case, try not to access them or check them for null value before doing anything else.
If the above is not your case, could you please send us some code extracts or a repro example so that we can better understand what your scenario is?
Let us know in case you need further help.
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