`makeTies` raises `KeyError` when bridging a string voice id absent from the next measure

[rdebroiz]

music21 version

master

Operating System(s) checked

any

Problem summary

makeRests(fillGaps=True) fills a gap in a voice with a rest of the gap's full length,
unclamped. If that rest crosses a barline, makeTies splits it and looks for the same voice in the next measure. When the voice is not there, the lookup raises instead of falling back, and the export dies.

Steps to reproduce

from music21 import note, stream

p = stream.Part()
m1 = stream.Measure()
v = stream.Voice(id='6')
v.insert(0, note.Note(quarterLength=1))
v.insert(5, note.Note(quarterLength=1))   # gap 1->5 runs past the 4.0 barline
m1.insert(0, v)
m2 = stream.Measure()
m2.insert(0, stream.Voice(id='1'))        # has a voice, but not '6'
p.append(m1)
p.append(m2)
p.write('musicxml')                       # KeyError: '6'

More information

With id=6 and id=1 as integers, the same score exports fine.

if not isinstance(vId, int):
    dst = mNext.voices[vId]          # subscript: raises on a miss
else:
    dst = mNext.getElementById(vId)  # returns None on a miss
...
if dst is None:
    dst = mNext                      # fallback the str branch never reaches

Suggested fix, so the string branch reaches the fallback that already exists:

dst = mNext.voices[vId] if vId in [voice.id for voice in mNext.voices] else None

Related:

#922 added this makeTies call to makeRests to "split rests at measure boundaries"
#890 asks "what happens if it ends and then begins again?"