Article relates to
RadEditor for ASP.NET AJAX
One alternative approach to fix the problem is to declare on your page (out of any AJAX panel):
Then, just set the OnClientCommandExecuting property of RadEditor, e.g.
<telerik:RadEditor OnClientCommandExecuting="OnClientCommandExecuting" .../>
Copyright © 2016, Progress Software Corporation and/or its subsidiaries or affiliates. All Rights Reserved.
Progress, Telerik, and certain product names used herein are trademarks or registered trademarks of Progress Software Corporation and/or one of its subsidiaries or affiliates in the U.S. and/or other countries. See Trademarks or appropriate markings.