NCRoomsManager is retained when opening threads from conversation list

[SystemKeeper]

Steps to reproduce

1. In conversation list tab "Threads"
2. Open a thread
3. Close it via "Close" button

-> NCRoomsManager is retained, as the NCRoomController is not removed. The issue comes from the return statement introduced at d00bf31. I think we can only return here if it's a threadview controller that is shown from a chat view, but not without it?

main @ 3abb177