Pushing Apply and then Submit on the Style Edit Page causes:
Mailing list discussion: https://sourceforge.net/p/geoserver/mailman/message/35658949/
The error can be reproduced on the default point style.
The issue is that the Abstract Style Page initializes a model object for the Legend, but effectively only during the constructor call – AbstractStylePage(StyleInfo style). The "apply" button kicks off some logic that nulls out the Legend object if the user didn't provide any legend-related input, but doesn't refresh the page (and thus doesn't re-initialize the legend object).
When the user then pushes submit, the null legend model breaks the form submission.