Clients is null

HealthMonitor

namespace MC.Bureau.Adaptation.Service
{
    public class HealthMonitor :IWantToRunAtStartup
    {
        private string url = "http://localhost:8081/";
        private Server _server;
        private HealthMonitorHub _healthMonitor;

        public ITransport Transport { get; set; }
      
        

        public void Run()
        {
            Transport.TransportMessageReceived += OnTransportMessageReceived;

            _server = new Server(url);
            _healthMonitor = new HealthMonitorHub();
            _server
                .MapHubs();
       

            _server.Start();
            
            
        }

        public void Stop()
        {
            throw new NotImplementedException();
        }

        private void OnTransportMessageReceived(object sender, TransportMessageReceivedEventArgs EventArgs)
        {
            _healthMonitor.Send(Encoding.Default.GetString(EventArgs.Message.Body));
        }
    }

    public class HealthMonitorHub: Hub
    {
        public void Send(string message)
        {
            
            Clients.SendMessage(message);
        }
    }
}