Issue collector edition fails silently when "Allow unassigned issues" is turned off

Issue Summary

You cannot create or edit issue collectors if the option Allow unassigned issues is set to OFF in the general settings. That happens because the Assignee field is set as required by default, but it cannot be used in the issue collector.

The problem is that when you have that option disabled, and you try to perform one the mentioned actions with an issue collector, no error message will be displayed. The issue collector will just not be updated.

Steps to Reproduce

• Turn the option Allow unassigned issues off in the general settings: https://<INSTANCE NAME>.atlassian.net/secure/admin/EditApplicationProperties!default.jspa
• Attempt to edit an existing issue collector;
  • E.g. try to add a custom field to its form.

Here's a recording showing the behaviour: Issue collector edition fail.mp4

Expected Results

An error message will be displayed, stating the reason why the issue collector was not updated.

Actual Results

The issue collector just won't be updated, and no error message will be displayed.

Workaround

None at the moment.