The only known case, that leads to the described error is when the RadEditor control is integrated with third-party controls that cannot modify the control's layout.
As you can see in such cases this error is quite self-descriptive and it means that there are incompatibility issues that breaks the desired integration.
I suggest using an alternative approach that does not causes such issues or providing details about the expected functionality, so that we could recommend you an alternative approach.
If it appears that this problem can be isolated without third-party controls, please provide more details about the steps leading to the problem, so that i could be able to investigate this on my end.
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