I am quoting the answer of my colleague.
"This error could occur if you are setting a column editor control ID in the code-behind (as shown in the following help article
). If this is the case, please try removing the code where ID of the editor control is set and see if it solves the issue for you. In previous versions of our RadControls, the editor controls were without ID's, but in our latest versions an ID is given to every editor control and changing that ID would lead to the error you are receiving."
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