This happened to me on community-release on Firefox/Mac and to Elias on community-daily on Chrome/Mac at the same time. We don't know what triggers it, but it seems to only affect people who are currently viewing a DM channel since Christopher's browser didn't crash at the same time.
The crash is caused by a combination of the websocket event for deleting a channel being sent to all users, causing them to store an incomplete channel object. It's possible that this might cause other side effects, but most notably, it causes getRedirectChannelNameForTeam to error out making the app crash.
1) Log into Mattermost as two different users
2) Have one user create or join a private channel
3) Delete that channel
Observed: The other user (and anyone that hasn't loaded the private channel) will have their Mattermost instance crash
Expected: Nothing happens for the other users
Tested and passed on 5.12.1-rc1
App doesn’t crash for the other user
No console errors
Labeled ‘done-5.12.1’ and added v5.13 to the fix version.
Verified the fix on 5.13.
Added a test to the release spreadsheet and labelled the PRs. Closing.