I am posting my reply from the support ticket you have opened for this issue, in case anyone else encounters a similar problem. If you still need assistance for resolving the issue, please write back to us in the ticket.
I looked at the provided screenshots and description but cannot see an obvious reason for this error. Can you send us the code that defines the RadFilter and any other related code? We would need to reproduce the issue in order to tell you what causes it.
As debugging steps you could try:
1) If you have AJAX in this scenario, try temporarily disabling it to see if this changes the page behavior.
2) If you create custom editors, make sure you follow the exact sequence of actions, as our samples (for example, this demo).
3) If you are adding the CheckBox and RadComboBox to a part of the RadFilter control, try taking them out and see if the error goes away. In such case, if it is not an option to leave them out, make sure you create them as a part of a custom field editor and initialize them correctly.
the Telerik team
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