How to use session outside a controller

I have a Signalr Hub called NotificationHub that handles sending new notification to coected clients. The NotificationHub class uses a NotificationManager class to retrieve notifications data. Now, I want to be able to use a session to store the last time a new notification has been accessed but when using HttpContext.Current.Session["lastRun"] in NotificationManager I get a NullReferenceException. To clarify more, here is some of the codes of both classes:

NotificationHub

[HubName("notification")]
    public class NotificationHub : Hub
    {
        private NotificationManager _manager;
        private ILog logger = LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);

        public NotificationManager Manager
        {
            get { retu _manager; }
            set { _manager = value; }
        }

        public NotificationHub()
        {
            _manager = NotificationManager.GetInstance(PushLatestNotifications);
        }

        public void PushLatestNotifications(ActivityStream stream)
        {
            logger.Info($"Adding {stream.TotalItems} notifications ");
            Clients.Caller.addLatestNotifications(stream);
        }
//.....
}

NotificationManager

 public class NotificationManager
        {
            private static NotificationManager _manager;
            private DateTime _lastRun;
            private DbUpdateNotifier _updateNotifier;
            private readonly INotificationService _notificationService;
            private readonly Action<ActivityStream> _dispatcher;
            private long _userId;
            private IUnitOfWork unitOfWork;
            public NotificationService NotificationService => (NotificationService)_notificationService;
            public DbUpdateNotifier UpdateNotifier
            {
                get { retu _updateNotifier; }
                set { _updateNotifier = value; }
            }

            public static NotificationManager GetInstance(Action<ActivityStream> dispatcher)
            {
                retu _manager ?? new NotificationManager(dispatcher);
            }



            private NotificationManager(Action<ActivityStream> dispatcher)
            {
                _userId = HttpContext.Current.User.Identity.CurrentUserId();
                _updateNotifier = new DbUpdateNotifier(_userId);
                _updateNotifier.NewNotification += NewNotificationHandler;
                unitOfWork = new UnitOfWork();
                _notificationService = new NotificationService(_userId, unitOfWork);
                _dispatcher = dispatcher;

            }



         private void NewNotificationHandler(object sender, SqlNotificationEventArgs evt)
                {

                  //Want to store lastRun variable in a session here
                    var notificationList = _notificationService.GetLatestNotifications();
                    _dispatcher(BuilActivityStream(notificationList));
                }
           //....
}

I want to able to store the value of lastRun to a session to that I can retrieve the next time a new notification arrives. How can I achieve that?