Ensure that mailbox setup can proceed after activity was destroyed

There is still a minor issue that you need to navigate back two times after landing in this corner case (because fragments get restored, but our isSetup() check posts an event that shows the initial fragment again). However, these activity got destroyed cases are super tricky and here I think fixing this as well isn't worth the effort. Not as important as the user not being able to navigate forward.

Closes #2342 (closed)