MR - Reply to a comment doesn't show the editor

Summary

When replying to an MR comment, the text filed disappear.

Steps to reproduce

We don't have proper steps to reproduce. This happens with the user's project.

Example Project

What is the current bug behavior?

What is the expected correct behavior?

Relevant logs and/or screenshots

Browser's Console Outputt

TypeError: Cannot read property 'base_sha' of null
    at a.markdownPreviewPath (pages.projects.merge_requests.show.c207c5f0.chunk.js:13)
    at gn.get (main.3dc33e63.chunk.js:63)
    at gn.evaluate (main.3dc33e63.chunk.js:63)
    at a.markdownPreviewPath (main.3dc33e63.chunk.js:63)
    at a.<anonymous> (pages.projects.merge_requests.show.c207c5f0.chunk.js:13)
    at a.t._render (main.3dc33e63.chunk.js:63)
    at a.r (main.3dc33e63.chunk.js:63)
    at gn.get (main.3dc33e63.chunk.js:63)
    at new gn (main.3dc33e63.chunk.js:63)
    at main.3dc33e63.chunk.js:63

TypeError: Cannot read property 'focus' of undefined
    at a.mounted (pages.projects.merge_requests.show.c207c5f0.chunk.js:13)
    at zt (main.3dc33e63.chunk.js:63)
    at nn (main.3dc33e63.chunk.js:63)
    at Object.insert (main.3dc33e63.chunk.js:63)
    at E (main.3dc33e63.chunk.js:63)
    at a.__patch__ (main.3dc33e63.chunk.js:63)
    at a.t._update (main.3dc33e63.chunk.js:63)
    at a.r (main.3dc33e63.chunk.js:63)
    at gn.get (main.3dc33e63.chunk.js:63)
    at gn.run (main.3dc33e63.chunk.js:63)

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