Pulsewatermeter Mysensors-mqttgw-openhab

Dear forum member.
I have an arduino/nrf24 sensor which request an previous count in v_var1. How to setup the correct item(s) in default.items and howto create a rule that reply with the latest stored value from persistence.

Found a lot information on the net, but not the right thing.

Number previous_water_value "Vorige waarde [%.3f m3]"  <water> (MqttTesting)   {mqtt="<[pimosquitto:mygateway1-out/77/0/2/0/24:state:default]"}*

Above the request (mysensors function 2 )
Below the response in the log

given new state is NULL, couldn't post update for 'previous_water_value'

Any help welcome ! Running OH 1.84