No error while I get 500 response code on updating username

Summary

I tried to update the username of my gitlab account. I didn't got any error instead got an error block in the top of the page.

Steps to reproduce

Try updating the username. Then try to revert to old username.

Example Project

What is the current bug behavior?

image

What is the expected correct behavior?

Some valid error message or success response after updating the username.

Relevant logs and/or screenshots

image

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info 
(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check 
(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)


(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)


(we will only investigate if the tests are passing)

Possible fixes