Could you try calling the editor's onParentNodeChanged() method as adivsed here: http://www.telerik.com/help/aspnet-ajax/window-troubleshooting-radeditor-in-radwindow.html
editor = $find(
"<%= emailPreview.ClientID %>"
"Test email preview"
// you can try replacing it with repaint() if it does not help
You can also consider using a RadWindow instead of the other dialog you are using right now.
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