removing an approver on the edit page causes 500 error

Summary

When removing an approver on a merge request, the application throws a 500 error.

Steps to reproduce

1. Add an approver to a merge request
2. click the remove button the right side of approvers list

What is the current bug behavior?

A 500 error is thrown

What is the expected correct behavior?

An approver should be removed - but ideally nothing.

Output of checks

This bug happens on GitLab.com

Possible fixes

When clicking to edit a merge request, any changes should be persisted when Save Changes is clicked. Currently when removing an approver, a request is sent immediately and the error occurs.

I'd like to try and tackle this problem. My suggestion would be to remove an approver once Save Changes is clicked and not as an ajax request.