Make sure account is valid when merging topics

Ensures a valid account, or nil, is used when merging topics. 
Rolls back changes in 8893587 that could block using the reader when no one was logged in.

Fixes #2083 (again)
Minor note: I couldn't manage to crash the app (which is great!). But this code executes even if you're effectively logged off. It doesn't crash or whatsoever. 

If that behavior is expected, i'd say :shipit: !