Hi,
Since there are no notes in the song, that's what's giving the error. Notation Musician is designed to open and transcribe MIDI files to sheet music, so there would be no point in opening an empty file, but it could let you know that the file is empty more gracefully
(Notation Composer "inherits" this ungraciousness as well.)
If you put a single note in the song, you should then be able to open it up in Notation Composer and further edit the song (though you can set up song files in Notation Composer as well). I'll write up this improvement in behavior.
Thanks!
Sherry