Error 500 if user has no voicemail on calld

If i have no voicemail and i'm using the api /users/me/voicemails, there is a 500 error.

2019-11-29 14:30:36,241 [15312] (ERROR) (flask.app): Exception on /1.0/users/me/voicemails [GET]
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/flask/app.py", line 1813, in full_dispatch_request
rv = self.dispatch_request()
File "/usr/lib/python3/dist-packages/flask/app.py", line 1799, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "/usr/lib/python3/dist-packages/flask_restful/_init_.py", line 458, in wrapper
resp = resource(*args, **kwargs)
File "/usr/lib/python3/dist-packages/flask/views.py", line 88, in view
return self.dispatch_request(*args, **kwargs)
File "/usr/lib/python3/dist-packages/flask_restful/_init_.py", line 573, in dispatch_request
resp = meth(*args, **kwargs)
File "/usr/lib/python3/dist-packages/xivo/rest_api_helpers.py", line 28, in wrapper
return func(*args, **kwargs)
File "/usr/lib/python3/dist-packages/wazo_calld/http.py", line 30, in wrapper
return func(*args, **kwargs)
File "/usr/lib/python3/dist-packages/xivo/mallow_helpers.py", line 27, in wrapper
return func(*args, **kwargs)
File "/usr/lib/python3/dist-packages/xivo/auth_verifier.py", line 111, in wrapper
return func(*args, **kwargs)
File "/usr/lib/python3/dist-packages/wazo_calld/plugins/voicemails/resources.py", line 96, in get
voicemail_id = _get_voicemail_id_from_request(self._auth_client, self._voicemails_service)
File "/usr/lib/python3/dist-packages/wazo_calld/plugins/voicemails/resources.py", line 268, in _get_voicemail_id_from_request
return voicemails_service.get_user_voicemail_id(user_uuid)
File "/usr/lib/python3/dist-packages/wazo_calld/plugins/voicemails/services.py", line 74, in get_user_voicemail_id
return user_voicemail_conf['id']
TypeError: 'NoneType' object is not subscriptable
2019-11-29 14:30:36,242 [15312] (INFO) (flask.app): response: (xxxxx) GET https://xxxxx/api/ctid-ng/1.0/users/me/voicemails 500