I have a cluster with two files, one has track number information and the other doesn't have any.

This crashes picard with:

Traceback (most recent call last):
 File "/usr/lib64/python3.6/site-packages/picard/ui/mainwindow.py", line 878, in view_info
   dialog = ClusterInfoDialog(cluster, self)
 File "/usr/lib64/python3.6/site-packages/picard/ui/infodialog.py", line 334, in _init_
   super()._init_(cluster, parent)
 File "/usr/lib64/python3.6/site-packages/picard/ui/infodialog.py", line 131, in _init_
   self._display_tabs()
 File "/usr/lib64/python3.6/site-packages/picard/ui/infodialog.py", line 135, in _display_tabs
   self._display_info_tab()
 File "/usr/lib64/python3.6/site-packages/picard/ui/infodialog.py", line 362, in _display_info_tab
   lines = ["%s %s - %s (%s)" % item for item in sorted(tracklist, key=sorttracknum)]
TypeError: '<' not supported between instances of 'int' and 'str'
Aborted (core dumped)