cockpit: tries to add empty domain

While testing an unstable image with plinth 0.16.0, I found this error in the log:

[2017-11-08 01:36:33,280] plinth.module_loader INFO     Module load order - ['apache', 'avahi', 'bind', 'cockpit', 'firewall', 'names', 'config', 'datetime', 'deluge', 'diagnostics', 'dynamicdns', 'ejabberd', 'first_boot', 'help', 'ikiwiki', 'infinoted', 'jsxc', 'letsencrypt', 'matrixsynapse', 'minetest', 'monkeysphere', 'mumble', 'networks', 'openvpn', 'pagekite', 'power', 'privoxy', 'quassel', 'radicale', 'repro', 'roundcube', 'security', 'snapshot', 'ssh', 'sso', 'storage', 'syncthing', 'tahoe', 'tor', 'transmission', 'ttrss', 'upgrades', 'users']
[2017-11-08 01:36:33,326] plinth.actions INFO     Executing command - ['sudo', '-n', '/usr/share/plinth/actions/cockpit', 'add-domain', '']
[2017-11-08 01:36:33,508] plinth.actions ERROR    Error executing command - ['sudo', '-n', '/usr/share/plinth/actions/cockpit', 'add-domain', ''], , Traceback (most recent call last):
  File "/usr/share/plinth/actions/cockpit", line 139, in <module>
    main()
  File "/usr/share/plinth/actions/cockpit", line 135, in main
    subcommand_method(arguments)
  File "/usr/share/plinth/actions/cockpit", line 104, in subcommand_add_domain
    origins = _get_origin_domains(aug)
  File "/usr/share/plinth/actions/cockpit", line 88, in _get_origin_domains
    return set(origins.split())
AttributeError: 'NoneType' object has no attribute 'split'