TODOs crash server when there's notification to removed issue

After recent update to Gitlab 8.6 and installing Todos extensions I started playing with it:

1. Logged in as user1 and created new issue in project, mentioning user2 in description
2. Logged in as user2 and verified that notification came, Todos counter started displaying 1 and Todo item appeared on Todos page
3. Closed and removed issue
4. Tried seeing /dashboard/todos page for user2 and got "500 Whoops, something went wrong on our end." instead.

Note that Todo counter for user2 still displays 1, so I believe that the Todo item wasn't removed during issue removal and it now causes mayhem when Gitlab tries displaying it.