For completeness, I will answer here, as well as your support ticket.
Currently, pages in cms edit mode do not have
HttpContext. RadControls require it to register with AjaxManager.
Therefore, you see an error. We have an interface,
Telerik.IEmptyControl, in Telerik.Framework.dll, that defines a message
that will be shown when the control is empty. The interface also tells
when the control is empty.
In this case, you will make the control "empty" if it is in edit mode.
We are working on providing a new page editor for the next version of
Sitefinity, which will not have this problem with HttpContext.
the Telerik team