As a teacher:

• Use Clean theme (with Boost the error doesn't appear).
• Create a Workshop activity, ensuring that you choose the following options:
• Grading strategy: Rubric
• Maximum number of overall feedback attachments: 1 (or higher)
• In the "Edit assessment form" step in the Setup phase, create a basic rubric using the "List" layout (in the "Rubric configuration" section at the end of the form)
• Press "Save and display" - you should see a notice:

  Notice: Undefined variable: type in /path/to/mymoodle/moodle/lib/pear/HTML/QuickForm/group.php on line 286

This also causes problems later when the same form (I believe) is used to assess a student's submission. It appears that part of the form is not rendered, as when you try to save the assessment an error stating You have to select one of these items is displayed, yet there are no items to select.