[Assignments] Prevent publishing a group assignment if groups are not specified

[NicoDurand]

Problem statement:

My students started submitting an assignment before I set the Group Set. Now Canvas doesn't let me change the groups, and only the student who submitted sees my comments and the grades.

Proposed solution:

The correct behavior should be: Don't allow Publishing a group assignment if the Group Set is not set.

User role(s):

instructor

[AllisonHowell]

This idea has been added to the Enhance assignment rules and functionality theme for further consideration.