ViewState is null in Monitor Node OPCUA in ASP.Net

I have monitor node in OPCUA and the data will auto update in Notification_MonitoredItem().

I use ViewState to save the data and timer tick (1s) to push data on web asp. But the ViewState always [null].

How to fix this or have any way to get data and show on web view.

Thanks

protected void Subcribe_Click(object sender, EventArgs e)
{
    if (Connect.myMonitoredItem != null && Connect.mySubscription != null)
    {
        try
        {
            Connect.myMonitoredItem = Connect.myClientHelperAPI.RemoveMonitoredItem(Connect.mySubscription, Connect.myMonitoredItem);
        }
        catch
        {
        }
    }
    try
    {
        //use different item names for correct assignment at the notificatino event
        Connect.itemCount++;
        string monitoredItemName = "myItem" + Connect.itemCount.ToString();
        if (Connect.mySubscription == null)
        {
            Connect.mySubscription = Connect.myClientHelperAPI.Subscribe(1000);
        }
        Connect.myMonitoredItem = Connect.myClientHelperAPI.AddMonitoredItem(Connect.mySubscription, SubcriptionID.Text, monitoredItemName, 1);
        Connect.myClientHelperAPI.ItemChangedNotification  += new MonitoredItemNotificationEventHandler(Notification_MonitoredItem);
        timer1.Enabled = true;

    }
    catch (Exception ex)
    {
        ClientScript.RegisterStartupScript(this.GetType(), "Alert", "mess('error','Opps...'," + ex.ToString() + ")", true);
    }
}

private void Notification_MonitoredItem(MonitoredItem monitoredItem, MonitoredItemNotificationEventArgs e)
{
    MonitoredItemNotification notification = e.NotificationValue as MonitoredItemNotification;
    if (notification == null)
    {
        return;
    }
    ViewState["Item"] = "Item name: " + monitoredItem.DisplayName;
    ViewState["Value"] = "Value: " + Utils.Format("{0}", notification.Value.WrappedValue.ToString());
    ViewState["Source"] = "Source timestamp: " + notification.Value.SourceTimestamp.ToString();
    ViewState["Server"] = "Server timestamp: " + notification.Value.ServerTimestamp.ToString();
}

protected void UpdateTimer_Tick(object sender, EventArgs e)
{

    if (ViewState["Item"] != null && ViewState["Value"] != null && ViewState["Source"] != null && ViewState["Server"] != null)
    {
        subscriptionTextBox.Text = ViewState["Item"].ToString();
        subscriptionTextBox.Text += ViewState["Value"].ToString();
        subscriptionTextBox.Text += ViewState["Source"].ToString();
        subscriptionTextBox.Text += ViewState["Server"].ToString();
    }
}