Could you send the link to your form and a screenshot of the error message to support@plumsail.com?
This way I can reproduce the issue and come up with a fix for you.
In Document Libraries, new forms are supported only for folder-derived content types, but it appears like you're trying to save a new form of a Document.
Could you describe your use case in more detail?
Maybe a default SharePoint new form would work better in this case?
@DaveD - I just completed this same update of webpart and widget via uploading a new App package. I am not saying that your experience is the same (my versions were quite a bit older than what you've shared), but curious if you're seeing other odd behaviors on your form? My JavaScript and CSS was impacted greatly after updating. Maybe you've figured out the problem already but can you comment out the offending block of JS and eliminate the error? I found that I had to rework my entire body of JS, to sort of reset the functionality. It was quite an effort and bet you're already beyond that point. Good luck.