Undefined offset error

Try duplicating/setting up a new form without any list fields first to see if the issue is still present. If the issue has gone away, try adding 1 “list” field and see if the error then returns.

Try also a combination of settings within the list field such as adding a label etc. It could well be a bug in the current version of Ninja Forms so if you can reproduce the issue, their support team will love to hear about it. Export the form and send it to them over at https://ninjaforms.com/contact/ so they can reproduce the issue and push out a fix.

As strange as it seems, other plugins can also conflict, so to be absolutely sure, deactivate all other plugins temporarily while testing.

If your form still works 100% then it might also be a good idea to turn off errors on the front-end of your website so customers do not see it, and instead, log them to your server.