Autocompletion of artifacts is throwing stack traces

Summary

When trying to autocomplete an artifact, BuildStream throws an exception

Steps to reproduce

bst artifact delete <TAB> <TAB>

Relevant logs and/or screenshots

buildstream@d09513488e77:~/project$ bst artifact delete Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_cas/cascache.py", line 183, in release_resources
    self._casd_process.wait(timeout=0.5)
  File "/usr/lib/python3.7/subprocess.py", line 990, in wait
    return self._wait(timeout=timeout)
  File "/usr/lib/python3.7/subprocess.py", line 1616, in _wait
    raise TimeoutExpired(self.args, timeout)
subprocess.TimeoutExpired: Command '['/usr/local/bin/buildbox-casd', '--bind=unix:/tmp/buildstreamkhholy_4/casd.sock', '/home/buildstream/.cache/buildstream']' timed out after 0.5 seconds

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/bin/bst", line 10, in <module>
    sys.exit(cli())
  File "/usr/local/lib/python3.7/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_frontend/cli.py", line 196, in override_main
    if main_bashcomplete(self, prog_name, partial(override_completions, args)):
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_frontend/complete.py", line 335, in main_bashcomplete
    do_complete(cmd, prog_name, override)
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_frontend/complete.py", line 325, in do_complete
    for item in get_choices(cli, prog_name, args, incomplete, override):
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_frontend/complete.py", line 294, in get_choices
    choices.extend(get_user_autocompletions(all_args, incomplete, ctx.command, cmd_param, override))
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_frontend/complete.py", line 243, in get_user_autocompletions
    incomplete=incomplete)
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_frontend/cli.py", line 185, in override_completions
    return complete_artifact(orig_args, args, incomplete)
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_frontend/cli.py", line 159, in complete_artifact
    return complete_list
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_context.py", line 193, in __exit__
    self._cascache.release_resources(self.messenger)
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_cas/cascache.py", line 189, in release_resources
    with cm:
  File "/usr/lib/python3.7/contextlib.py", line 112, in __enter__
    return next(self.gen)
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_messenger.py", line 168, in timed_activity
    self.message(message)
  File "/usr/local/lib/python3.7/dist-packages/buildstream/_messenger.py", line 123, in message
    assert self._message_handler
AssertionError

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