Could you please set the true
argument of the get_html method
and test again the result. When the true argument is set the get_html method will return the content via the editor's client-side functions.
By the way you can see the following forum thread on the subject which could be helpful for your scenario: Detect editor content change.
The code applies to RadEditor Classic, but you can see how to attached event handlers in RadEditor for ASP.NET AJAX here
the Telerik team