Commenting on MR diff doesn't work sometimes

Summary

It looks like under some unknown conditions, commenting on MR diffs doesn't work. This is intermittent error, and I wasn't able to find reproducible process for that.

Adding comment results in error:

application-9d7ddc5….js:490 Uncaught TypeError: Cannot read property 'destroy' of undefined

Notification about comment is sent, but comment is not visible in discussion.

Steps to reproduce

Comment on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4024#note_12008228

Reproduced by @alfredo1.

/cc @jschatz1