This is a server error which raises when you have not correctly referenced a control, most probably in your case the editor reference is null and you are trying to set the Content property. However, in order to be able to help, we need more information about your setup and your code. Would you please provide more information about it - your exact code on the client and on the server which causes the problem, tell us whether you use a MasterPage, etc. It will be best if you open a new support ticket and send us a simple, fully runnable reproduction demo along with reproduction steps.
the Telerik team