I believe that's the very same code I've suggested in my previous post. The third argument of the 
https://winscp.net/eng/docs/library_session_getfilestodirectory#syntax
You just explicitly pass the default
        Session.GetFilesToDirectory is optional and defaults to null:
https://winscp.net/eng/docs/library_session_getfilestodirectory#syntax
You just explicitly pass the default
null value.