NoneType' object is not iterable

[shaokun11]

I change the musicxml file from haendel_hallelujah.muscxml to mozart_requiem_kyrie_satb.musicxml which you provide in the data directory, it rase a error as below.(inference.py content is same as the inference.ipynb which just remove the matplotlib something )
Traceback (most recent call last): File "inference.py", line 129, in <module> data = get_data_from_musicxml('data/mozart_requiem_kyrie_satb.musicxml', 132, convert_stress=True) File "/home/user/python-project/mellotron/mellotron_utils.py", line 453, in get_data_from_musicxml events = track2events(v) File "/home/user/python-project/mellotron/mellotron_utils.py", line 285, in track2events events.extend(adjust_event(e)) File "/home/user/python-project/mellotron/mellotron_utils.py", line 235, in adjust_event return [[token, freq, start_time, end_time] for token in tokens] TypeError: 'NoneType' object is not iterable
what should I change something?