Stopping default onOK from dialog closing the dialog for CKEDITOR

I am developing a plugin and I need that when the user clicks OK on the dialog I have to do some other processing, so I want to stop the dialog from closing and show an error when the information is wrong.

I've tried to cancel, stop the event, but it didn't work.

onOk: function (event) {
            event.cancel();
            event.stop();
            return false;
        }