Unexpected behavior for MqttPairItem with retain

[spacemanspiff2007]

trafficstars

When a device sends a message with retain=True HABApp will automatically create an MqttItem for that Topic. However if the user wants a MqttPairItem this will raise an Error in `MqttPairItem.get_create_item(...)

It would be nice if this would still work, so there is a need for some concept.