Please, read my earlier post once again.
You should pass the ID of RadEditor to the ControlsToCheck property but not the string "description"; as you have set this.radspell.ControlToCheck = "description"
;, because you do not have such an editor with ID description on the page.
You should set
this.radspell.ControlToCheck = description.ClientID;
this.radspell.ControlToCheck = description.ID;
You can find an example with the ControlsToCheck property set in the codebehind in the following live demo: Overview
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