Cannot add milestone to an issue directly from a board

Summary

I can't assign a milestone to an issue directly from the Board view.

Steps to reproduce

On an Issues Board, click on a card (note the title) to make the panel on the right appear. Click on "edit" in the milestone section, select an existing milestone.

Note : It still works perfectly fine if I go into the detailed issue page and set the milestone from there.

Example Project

https://gitlab.com/nicolus/milestone-from-issue-board-bug/boards

What is the current bug behavior?

The spinning loader appears but never disappears.

There's an error in the javascript console : TypeError: o is undefined; can't access its "detail" property

If I reload the page, the issue is not affected to the milestone.

What is the expected correct behavior?

The milestone should be added to the issue, and the loader should disappear.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com