EDIT:- The following diagnosis is when the form widget is launched via Article Layout Builder.

line 25 - $element - Produces expected results when launched via layout.

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Form builder produces "undefined" message when trying to submit a form using the Article Layout Builder.

I've only had a brief look, I believe the bug lies within formbuilder/ajax.php

line 25 - $element looks to return false which prevents the if statement to process from line 26.

However, even if processed, line 31 $params, is not going to fetch the correct information.

Running latest commit, on Joomla 5.

I don't believe to be a recent issue as it's was mentioned in September

#798