Error can only concatenate str (not "NoneType") to str

[Martinvdm]

Getting this error in Home assistant 0.99.2.
Did not had the error before.

2019-09-23 21:11:54 ERROR (MainThread) [homeassistant.components.websocket_api.http.connection.140111991634192] can only concatenate str (not "NoneType") to str,
Traceback (most recent call last):,
  File "/usr/src/homeassistant/homeassistant/components/websocket_api/commands.py", line 130, in handle_call_service,
    connection.context(msg),,
  File "/usr/src/homeassistant/homeassistant/core.py", line 1234, in async_call,
    await asyncio.shield(self._execute_service(handler, service_call)),
  File "/usr/src/homeassistant/homeassistant/core.py", line 1259, in _execute_service,
    await handler.func(service_call),
  File "/usr/src/homeassistant/homeassistant/helpers/entity_component.py", line 213, in handle_service,
    self._platforms.values(), func, call, service_name, required_features,
  File "/usr/src/homeassistant/homeassistant/helpers/service.py", line 349, in entity_service_call,
    future.result()  # pop exception if have,
  File "/usr/src/homeassistant/homeassistant/helpers/service.py", line 371, in _handle_service_platform_call,
    await getattr(entity, func)(**data),
  File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run,
    result = self.fn(*self.args, **self.kwargs),
  File "/config/custom_components/ziggo-mediabox-next/mediabox_next.py", line 184, in turn_off,
    self.__send_key(MEDIA_KEY_POWER),
  File "/config/custom_components/ziggo-mediabox-next/mediabox_next.py", line 227, in __send_key,
    'eventType': 'keyDownUp',
  File "/config/custom_components/ziggo-mediabox-next/mqtt_client.py", line 90, in publish,
    _LOGGER.info("PUBLISH " + self._topic),
TypeError: can only concatenate str (not "NoneType") to str,