FIX: had accidentally removed an `on_mqtt_message` usage

CHANGELOG.md

@@ -3,7 +3,7 @@
 All notable changes to this project will be documented in this file.
 Given the high speed of development in the first phase, changes are being reported only starting from v0.20.2.
 
-## [0.22.7] - 2021-12-13
+## [0.22.8] - 2021-12-13
 
 ### Added
 

platypush/backend/mqtt/__init__.py

@@ -267,6 +267,7 @@ class MqttBackend(Backend):
                     tls_certfile: Optional[str] = None, tls_keyfile: Optional[str] = None, tls_version: Optional = None,
                     tls_ciphers: Optional = None, tls_insecure: bool = False, on_message: Optional[Callable] = None) \
             -> MqttClient:
+        on_message = on_message or self.on_mqtt_message()
         client_id = self._get_client_id(host=host, port=port, topics=topics, client_id=client_id, on_message=on_message)
         client = self._listeners.get(client_id)