Hello Bruno Michels,
In page edit mode Page is null and most probably this is the issue. You can escape the logic you have added only in edit mode and stop showing the error message by using the QueryString
if(HttpContext.Current.Request.QueryString[ "cmspagemode"] == "edit")
//here disable RadMenu scripts
if("edit".Equals(HttpContext.Current.Items["cmspagemode"] as string))
or implement IEmptyControntrol interface.
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