Indeed we were able to replicate the behavior you described. As a kind of workaround you may try to manually delete design assemblies which should resolve the issue by your side. Will you please give it a try and let me know how it goes? Furthermore even if the GridViewHeaderCell is not visible in Assets Library you may manually write its definition and it will appear.
We will do our best to provide a fix for this in our next official release.
All the best,
the Telerik team
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