Error: Request failed with status code 500 When Update Username

Summary

I see an flash message box (there is no message) when trying to update my username

Steps to reproduce

• I visited this link to update my username.
• I tried to change my username with aligoren.
• I clicked Update Username button and a modal opend.
• I clicked Update Username button again.
• I saw red message box but there is no any text
• I check JavaScript console on Firefox Quantum

What is the current bug behavior?

Username is not being updated because ajax request returns 500 error. I think there is a problem on https://gitlab.com/profile/update_username.json link.

What is the expected correct behavior?

For example my username didn't update for any reasons, I should see information message. I should see an information message no matter what and of course my username should be updated.

Relevant logs and/or screenshots

Unhandled promise rejection 
Error: Request failed with status code 500
Stack trace:
e.exports@https://assets.gitlab-static.net/assets/webpack/main.b31f4713.chunk.js:71:122831
e.exports@https://assets.gitlab-static.net/assets/webpack/main.b31f4713.chunk.js:101:59401
e.exports/</p[v]@https://assets.gitlab-static.net/assets/webpack/main.b31f4713.chunk.js:71:123986
a@https://assets.gitlab-static.net/assets/webpack/raven.880e36dd.chunk.js:1:15146

I didn't see any message

Output of checks

This bug happens on GitLab.com

Possible fixes

If the problem is about the backend, actually I have no idea because I haven't any experience about ruby. But if the problem is about front-end when request returns 500 or similar error, JavaScript code should handle it.