private async void GetSensors(TickTackHub hub)
 {
     if (hub.Id == null) return;
     Sensors.Clear();
     var sensors = await _hubService.GetSensors(hub.Id);
     //todo: replace by sensors.ForEach(Sensors.Add);
     foreach (var sensor in sensors)
     { 
         Sensors.Add(sensor);
     }
 }