Loading MusicXML file produces error

[j-w-moebius]

music21 version

9.1.0

Problem summary

Calling converter.parse on this MusicXML file raises an error.

Steps to reproduce

environment.set('autoDownload', 'allow')
c = converter.parse("https://www.shapenote.net/musicxml/24.mxl")

Expected vs. actual behavior

Raises the exception TypeError: unsupported operand type(s) for +=: 'NoneType' and 'int' at music21/musicxml/xmlToM21.py:5587.

More information

Suggested fix (solved the issue for this file):
Change line 5585 into
if mxOctaveChange is not None and diatonicStep is not None: