expanding diff in review - TypeError: Cannot read property 'key' of undefined

Summary

During code review, expanding some diffs (not all the diffs), we get a console error and rewiew feature is gone.

Steps to reproduce

  1. go to gitlab-runner!1218 (diffs)
  2. expand the diff on the line above (322)
  3. now you cannot add a new comment or complete an ongoing review (on EE)

Relevant logs and/or screenshots

Collapsed screencast

This screencast is on canary, but production has the same behavior.

output

TypeError: Cannot read property 'key' of undefined
    at Gn (vue.esm.js:5545)
    at vue.esm.js:5947
    at E (vue.esm.js:6047)
    at vue.esm.js:5921
    at E (vue.esm.js:6047)
    at vue.esm.js:5921
    at E (vue.esm.js:6047)
    at a.__patch__ (vue.esm.js:6210)
    at a.e._update (vue.esm.js:2777)
    at a.i (vue.esm.js:2895)
    at It.get (vue.esm.js:3265)
    at It.run (vue.esm.js:3340)
    at Ot (vue.esm.js:3096)
    at Array.<anonymous> (vue.esm.js:1925)
    at Ke (vue.esm.js:1844)

Output of checks

This bug happens on GitLab.com