AttributeError: 'list' object has no attribute 'obj_list'

[wbsoft]

Reported by a Frescobaldi user:

Frescobaldi: 2.18.2 -- Python: 2.7.11 -- python-ly: 0.9.3 -- Qt: 4.8.7 -- PyQt: 4.11.4 -- sip: 4.17 -- poppler: 0.39.0 -- python-poppler-qt: 0.24.0 -- OS: Darwin-14.5.0-x86_64-i386-64bit

Traceback (most recent call last):
  File "/Applications/Frescobaldi.app/Contents/Resources/lib/python2.7/frescobaldi_app/file_export/__init__.py", line 65, in exportMusicXML
  File "ly/musicxml/lymus2musxml.pyc", line 146, in musicxml
  File "ly/musicxml/xml_objs.pyc", line 78, in __init__
  File "ly/musicxml/xml_objs.pyc", line 99, in iterate_part
  File "ly/musicxml/xml_objs.pyc", line 106, in iterate_bar
AttributeError: 'list' object has no attribute 'obj_list'

[wvengen]

I'm getting this error with something as simple as this (python-ly 0.9.7):

\score {
  <<
    \new Voice { a1 }
    \addlyrics { \lyricmode { Foo } }
  >>
  \layout { }
}

When removing the lyrics, it works.

[wvengen]

May be related to #57.