I am facing problem when intiatizing Signal r from Mvc web controller?

I am facing an issue in signalr hub from MVC controller. Am facing the below error.

For reference I have added below code used

Controller:

public JsonResult Index(Guid id, string user)
        {
            HttpStatusCode statusCode;
           
            var hubContext = GlobalHost.ConnectionManager.GetHubContext<NotificationHub>();
           
            NotificationHub hub = new NotificationHub();
            hub.AddNotification(user +"| called");


            return Json("Success",JsonRequestBehavior.AllowGet);
        }
Hub:

public async Task AddNotification(string data) {

if (_connections.Count > 0 )
{
 foreach (var connectionId in _connections.GetConnections(_user))
   {
      await Clients.Client(connectionId).BroadcastNewNotification(_msg);
    }
 }                        
           

}

But Am getting “Object reference not set to an instance of an object” while Broadcast notification