Register form extra fields validation issue

Hi, After adding the first and last name to my registration form as explained on https://docs.thememylogin.com/article/62-adding-extra-registration-fields I’ve found a bug.

If you leave a field blank and submit (eg last name), it shows the error which is great. But then if you fill in the last name and resubmit, there is a JS error and the form doesn’t continue.

The page I need help with: [log in to see the link]