How do I update progress bar value from background

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.Primitives;
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;
using System.Reflection;

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


        }

     Uri uri;
       

       static int row = 0;


        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;
        }
        
        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"));
            }
        }
     static public int p;
        private async Task Start( 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())
            {
                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());
                            
                            p = per;
                            // change(p);
                          //  MakeButton2();
                        }
                        await fs.FlushAsync();
                    }
                }
                           
              
            }
        }

        public Task Start()
        {
            _allowedToRun = true;
           
            return Start( BytesWritten);
        }
        
        public void Pause1()
        {
            _allowedToRun = false;
        }

        void MakeButton2()
        {
            Button b2 = new Button();
            //  b2.AddHandler(Button.ClickEvent, new RoutedEventHandler(Onb2Click2));
            // MessageBox.Show("HI");
         b2.Click += new RoutedEventHandler(Onb2Click2);
           // b2.AddHandler(Button.ClickEvent, new RoutedEventHandler(Onb2Click2));
           // Onb2Click2(null, RoutedEventArgs.Empty);
           // Onb2Click2(this, new RoutedEventArgs());

           //Onb2Click2(new object(), new RoutedEventArgs());

        }
       public void Onb2Click2(object sender, RoutedEventArgs e)
        {
           //    Button b = b2(sender);
           // MessageBox.Show("Hi");
          // ProgressBar pb= e.Source as ProgressBar;
            pb.Value = p;

            MessageBox.Show(p + "");
        } 
        void OnLoad(object sender, RoutedEventArgs e)
        {
            
          //  pb.Value = p;
            MessageBox.Show(p + "");

        }

        public void change (  int i)
        {
           
                MessageBox.Show(i + "");
           
            
           // pb.Value = i;

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

               Onb2Click2(sender, e);
            }
            catch (Exception ex) { MessageBox.Show(ex.Message); }
        }

        
    }


}

object reference not set to an instance