This function does not perform a postback, which means the origin of the validation error is somewhere else in your code. What I can suggest is making sure that the button that is used to call it does not perform a postback at all. Usually this is done by adding a return false; statement for the OnClientClick handler of the control.
I would also like to note that the original issue is not a problem with the control, but a direct consequence of the comfortable way radopen, radalert, radconfirm and radprompt can be used directly, just like the browser dialogs can.
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