I: 22:54:30,192 /usr/lib/python3.6/site-packages/picard/tagger.debug:234: Debug mode off
{{ Traceback (most recent call last):}}
{{ File "/usr/lib/python3.6/site-packages/picard/webservice/_init_.py", line 426, in _process_reply}}
{{ self._handle_reply(reply, request)}}
{{ File "/usr/lib/python3.6/site-packages/picard/webservice/_init_.py", line 413, in _handle_reply}}
{{ handler(document, reply, error)}}
{{ File "/usr/lib/python3.6/site-packages/picard/disc.py", line 71, in _lookup_finished}}
{{ dialog = CDLookupDialog(releases, self, parent=self.tagger.window)}}
{{ File "/usr/lib/python3.6/site-packages/picard/ui/cdlookup.py", line 66, in _init_}}
{{ item.setText(3, myjoin(countries))}}
{{ File "/usr/lib/python3.6/site-packages/picard/ui/cdlookup.py", line 55, in myjoin}}
{{ return "\n".join(l)}}
{{ TypeError: sequence item 0: expected str instance, NoneType found}}

One of the values in sequence 'l' is not a string. Possible fix is to use filter:

53 if self.releases:
{{ 54 def myjoin(l):}}
{{ 55 return "\n".join(filter(None,l))}}