Cannot place fieldsets within Accordion Item or Detail

Problem/Motivation

Steps to reproduce. I am getting an "undefined index" error in the reduceOrder() method within the field UI whenever I try to put a Fieldset field group within either an Accordion, an Accordion Item or a Detail field group. I have attached a screenshot of the error that I'm getting. Each fieldset group that I'm trying to position within an Accordion or Detail consists of a small number of fields (3 or 4) that I display in a row on my form. To be clear, I can create the Accordion, Accordion Item, or Detail field group; the error occurs when I try to place fieldsets within that group.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes