C#

CshDeposit5.xaml.cs
--------------------------------
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.IO.Ports;
using System.Windows.Threading;


namespace NWCDM
{
    public partial class CshDeposit5 : Page
    {
    private System.Windows.Threading.DispatcherTimer dispatcherTimer = new System.Windows.Threading.DispatcherTimer();
    private static int i; 


    public CshDeposit5()
    {
        InitializeComponent();
    }


    private void Page_Loaded(object sender, RoutedEventArgs e)
    {
        i = Global.xDropTime; //Stipulated Time for the object to pass through before timeout - declared in Global.cs

        TimeLeft.Text = i.ToString();

        dispatcherTimer.Tick += dispatcherTimer_Tick;
        dispatcherTimer.Interval = new TimeSpan(0, 0, 1);

        Global.xController.WriteLine("t"); //serialport declared in Global.cs file


        dispatcherTimer.Start();

        SerialPort sp2 = Global.xController; //Declared in Global.cs as serial port

        sp2.DataReceived += new SerialDataReceivedEventHandler(DataReceivedHandler );
    }


    public void DataReceivedHandler(object sender, SerialDataReceivedEventArgs e)
    {

    SerialPort sp = (SerialPort)sender;
    string indata = sp.ReadExisting();
    

    Dispatcher.Invoke(DispatcherPriority.Send, new Action(() => DoTrans(indata))); 

    }


    private void DoTrans(string x){

    if (i >= 0)
    {
        if (x.IndexOf("1") >= 0)
        {
            dispatcherTimer.Stop();

            Dispatcher.BeginInvoke(new System.Threading.ThreadStart(delegate
            {
                NavigationService.Navigate(new Uri("TmpMsg.xaml", UriKind.RelativeOrAbsolute)); //Throws Null exception 2nd time it comes here
            }));
        }
        else
        {
            if (x.IndexOf("0") >= 0)
            {
                Dispatcher.BeginInvoke(new System.Threading.ThreadStart(delegate
                {
                    NavigationService.Navigate(new Uri("TmpErrorMsg.xaml", UriKind.RelativeOrAbsolute)); //Throws Null exception 2nd time it comes here
                    }));
            }
        }
    }
}


private void dispatcherTimer_Tick(object sender, EventArgs e)
{
    if (i >= 0)
    {
        if (Global.VarLang == 0)
        {
            TimeLeft.Text = i.ToString();
        }
        i--; 
    }
    else
    {
        dispatcherTimer.Stop();

        Global.xController.WriteLine("r"); //Shutter Gate Closes after timeout

        Dispatcher.BeginInvoke(new System.Threading.ThreadStart(delegate
        {
        NavigationService.Navigate(new Uri("TmpErrorMsg.xaml", UriKind.RelativeOrAbsolute)); //No Error whatsoever
        })); 

        }

      }


    }
}