Files not editable in Web IDE. Error "Error setting up Web iDE, please try again"

Summary

When I edit files on an active MR, the web IDE presents an error. Also I cannot edit the files. If I close out the files and reopen within the Web IDE, then they are editable.

Steps to reproduce

1. Create branch and MR within the IDE
2. Edit files and commit to branch
3. Open Web IDE from MR.
4. Error presented at the top
5. Select files that were previously edited. Cannot open to edit.

Example Project

!75790 (closed)

What is the current bug behavior?

• Error at top "Error setting up Web iDE, please try again"
• Files not editable

What is the expected correct behavior?

• Files should be editable

Relevant logs and/or screenshots

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