How to update progress bar value in WPF from backend?

Question

I am developing a download manager, and I want to implement a progress bar, I have added that and it is working fine with webclient but now I am using HttpWebRequest and I am updating progress bar value but it is not updating sometimes it throws an exception "object reference not set to an instance" What can I do?
I am attaching my code Please Help!!
Thank in Advance

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.IO;

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.Shapes;
using System.ComponentModel;
using System.Windows;
using System.Net;
using System.Threading;


namespace DownloadManager
{
    /// <summary>
    /// Interaction logic for Download.xaml
    /// </summary>
    public partial class Download : Window
    {
        public Download()
        {
            InitializeComponent();

        }
      //  Download d1 = new Download();

        public Download(string url, string p)

        {

            Thread thread = new Thread(() => download(url,p));
            thread.Start();


        }

        Uri uri;
        static WebClient client = new WebClient();


       static int row = 0;


        double Percentage;

                Status.Content = "Downloading.";
                Status.Content = "Downloading..";
                Status.Content = "Downloading...";

            if (pb.Value == 100)
            {
                row++;
                Status.Content = "Completed";
                Close();


            }

        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
        }



        private volatile bool _allowedToRun;
        private string _source;
        private string _destination;
        private int _chunkSize;
        Download d1 ;

        private Lazy<int> _contentLength;

        public int BytesWritten { get; private set; }
        public int ContentLength { get { return _contentLength.Value; } }

        public bool Done { get { return ContentLength == BytesWritten; } }

        public Download(string source, string destination, int chunkSize)
        {
            _allowedToRun = true;

            _source = source;
            _destination = destination;
            _chunkSize = chunkSize;
            _contentLength = new Lazy<int>(() => Convert.ToInt32(GetContentLength()));

            BytesWritten = 0;
        }
        public int p0=10;
        private long GetContentLength()
        {
            var request = (HttpWebRequest)WebRequest.Create(_source);
            request.Method = "HEAD";

            using (var response = request.GetResponse())
                return response.ContentLength;
        }
        public event PropertyChangedEventHandler PropertyChanged;

        private int _Progress;
        public int Progress
        {
            get
            {
                return _Progress;

            }
            set
            {
                _Progress = value;
                PropertyChanged(this, new PropertyChangedEventArgs("Progress"));
            }
        }
        private async Task Start(object sender, int range)
        {
            if (!_allowedToRun)
                throw new InvalidOperationException();

            var request = (HttpWebRequest)WebRequest.Create(_source);
            request.Method = "GET";
            request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; .NET CLR 1.0.3705;)";
            request.AddRange(range);

            using (var response = await request.GetResponseAsync())
            {
            d1 = new Download();
                using (var responseStream = response.GetResponseStream())
                {
                    using (var fs = new FileStream(_destination, FileMode.Append, FileAccess.Write, FileShare.ReadWrite))
                    {
                        while (_allowedToRun)
                        {
                            var buffer = new byte[_chunkSize];
                            var bytesRead = await responseStream.ReadAsync(buffer, 0, buffer.Length);

                            if (bytesRead == 0) break;

                            await fs.WriteAsync(buffer, 0, bytesRead);
                            BytesWritten += bytesRead;
                            double receive = double.Parse(BytesWritten.ToString());

                            double Filesize = 5761048;
                            double Percentage = receive / Filesize * 100;

                            // a1 = int.Parse(Math.Truncate(Percentage).ToString());
                            //pb.Value = int.Parse(Math.Truncate(Percentage).ToString());
                            //progress(int.Parse(Math.Truncate(Percentage).ToString()));
                            // MessageBox.Show(int.Parse(Math.Truncate(Percentage).ToString()) + "Done");
                            int per = int.Parse(Math.Truncate(Percentage).ToString());
                            Download d = new Download();
                            try
                            {
                                d.p0 = per;

                            }
                            catch (Exception ex) { MessageBox.Show(ex.Message); }
                        }

                        await fs.FlushAsync();
                    }
                }


            }
        }

        public Task Start()
        {
            _allowedToRun = true;
            Download download = new Download();
            return Start( download,BytesWritten);
        }

        public void Pause1()
        {
            _allowedToRun = false;
        }

        public void change (object sender, RoutedEventArgs e)
        {
            pb.Value = p0;
          //  Pause_Click(sender, e);
        }
        private void Pause_Click(object sender, RoutedEventArgs e)
        {
            //change();

        }


    }


}