Test builds can't be closed after enabling mailbox flag

The following discussion from !302 (merged) should be addressed:

• @sebkur started a discussion: (+4 comments)

  maybe I was a bit too quick with this one. I just ran the app to see if groups also appear in the UI. They do, which is fine, but I also noticed the app does no longer shut down properly using the X window icon. Maybe the mailbox enabled does something unexpected. First we should find out if it's related to mailbox enabling or groups I guess.

This does not happen for release builds it seems. So a test specific issue.