Apologies if someone else has spotted this but when duplicating a form by copying the With Action definition from the original and pasting it into the new form, if the word "unique" is in the script, an "null" error message will be shown.

If you remove the existence of the word "unique" from the xml, the script will then save correctly. Perversely, once the script has been saved, you can put the "unique" word back and the script will still save ok.