I'm afraid that with the given details, I'm not sure what may be causing the described behavior. Are you trying to set manually focus to any control on the page? This error usually occurs when focus is called for a nonvisible html element.
It will be highly appreciated if you can send us (attached to a formal support ticket) a small working project in which this behavior can be reproduced. Thus we will be able to gather more details about your scenario and get back to you with resolution.
the Telerik team
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