C#

  using System;
using System.Drawing;
using System.Windows.Forms;
using Spire.Barcode;

namespace camera2
{
    public partial class Form2 : Form
    {
        Bitmap bitmap;
        //

<pre>
    public Form2()
    {
        InitializeComponent();
    }

    private void printDocument1_PrintPage(object sender, System.Drawing.Printing.PrintPageEventArgs e)
    {
      //  using System.Drawing.Printing;
        e.Graphics.DrawImage(bitmap, 0, 0);
    }

    private void CaptureScreen()
    {
        Graphics myGraphics = this.CreateGraphics();
        Size s = this.Size;
        bitmap = new Bitmap(s.Width, s.Height, myGraphics);
        Graphics memoryGraphics = Graphics.FromImage(bitmap);
        memoryGraphics.CopyFromScreen(this.Location.X, this.Location.Y, 0, 0, s);
    }
    private void print_Click(object sender, EventArgs e)
    {
        PrintDialog pDialog = new PrintDialog();
        pDialog.AllowSomePages = true;
        if (pDialog.ShowDialog(this) == DialogResult.OK)
        // this.printDoc.Print();
        {
           // Front card panel 1 
            Panel panel = new Panel();
            panel1.Size = new System.Drawing.Size(600,600);
            this.Controls.Add(panel);
            Graphics grp = panel.CreateGraphics();
            Size formSize = this.ClientSize;
            bitmap = new Bitmap(formSize.Width, formSize.Height, grp);
            grp = Graphics.FromImage(bitmap);
            Point panelLocation = PointToScreen(panel.Location);
            grp.CopyFromScreen(panelLocation.X, panelLocation.Y, 0, 0, formSize);
            printPreviewDialog1.Document = printDocument1;
            printPreviewDialog1.PrintPreviewControl.Zoom = 1;
            printPreviewDialog1.ShowDialog();

            // back card panel 2

            Panel panel2 = new Panel();
            panel2.Size = new System.Drawing.Size(600, 600);
            this.Controls.Add(panel);
            Graphics grp1 = panel.CreateGraphics();
            Size formSize1 = this.ClientSize;
            bitmap = new Bitmap(formSize.Width, formSize.Height, grp);
            grp1 = Graphics.FromImage(bitmap);
            Point panelLocation1 = PointToScreen(panel2.Location);
            grp.CopyFromScreen(panelLocation1.X, panelLocation1.Y, 0, 0, formSize1);
            printPreviewDialog1.Document = printDocument1;
            printPreviewDialog1.PrintPreviewControl.Zoom = 1;
            printPreviewDialog1.ShowDialog();

        }
    }

private async void FindAllBTN_Click(object sender, EventArgs e)
{
  progressBar.Minimum = 0;
  if (!isRunning)
  {
    OutputTB.Hide();
    OutputRTB.Show();
    FlipNextAllCKB.Checked = true;
    OutputRTB.Text = OutputTB.Text;

    FindAllBTN.Text = "Cancel";
    FindAllBTN.ForeColor = Color.Red;

    Progress<int> prog = new Progress<int>(SetProgress);
    m_cancelTokenSource = new CancellationTokenSource();

    try
    {
      await FindAllMatches(searchTB.Text, prog, m_cancelTokenSource.Token);
    }
    catch (OperationCanceledException)
    {
      //
    }
    finally
    {
      FindAllBTN.Text = "Find All";
      FindAllBTN.ForeColor = Color.Black;
      isRunning = false;
      m_cancelTokenSource = null;
      progressBar.Value = 0;
      replaceLTB.Focus();
    }
  }
  else
  {
    m_cancelTokenSource.Cancel();
  }

  return;

}

public Task FindAllMatches(string searchString, IProgress<int> prog, CancellationToken ct)
{
  return Task.Run(() =>
  {
    try
    {
      bool yesno = false;
      string source = "";

      Regex re;
      if (searchString == "")
      {
        OutputTB.AppendText("\r\nNeed RegEx Expression Input\r\n");
        return;
      }

      try
      {
        re = new Regex(searchString);
      }
      catch (ArgumentException e)
      {
        OutputTB.AppendText("\r\nEx Err " + e.ToString() + "\r\n");
        return;
      }

      source = OutputTB.Text;
      if (source == "")
      {
        OutputTB.AppendText("Need Source Text Input");
        return;
      }

      OutputRTB.SelectionStart = 0;
      OutputRTB.SelectionLength = OutputRTB.Text.Length;
      OutputRTB.SelectionBackColor = Color.White;

      MatchCollection matches = Regex.Matches(OutputRTB.Text, searchString);

      progressBar.Maximum = matches.Count;
      for(int i = 0; i < matches.Count; i++)
      {
        prog.Report(i);

        if (ct.IsCancellationRequested)
        {
          OutputRTB.AppendText($"\r\nCanceled at Loop {i}");
          OutputRTB.AppendText("\r\n");
          throw new OperationCanceledException(ct);
        }

        OutputRTB.SelectionStart = matches[i].Index;
        OutputRTB.SelectionLength = matches[i].Length;
        if (yesno)
        {
            OutputRTB.SelectionBackColor = Color.GreenYellow;
            yesno = false;
        }
        else
        {
            OutputRTB.SelectionBackColor = Color.Turquoise;
            yesno = true;
        }

      }
    }
    catch (Exception ex)
    {
      if(ex.ToString().Contains("OperationCanceledException"))
      {
        //
      }
      else
      {
        //
      }
    }
    finally
    {
      //
    }
  }, ct);

}

ee the end of this message for details on invoking 
just-in-time (JIT) debugging instead of this dialog box.

************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Windows.Forms.RichTextBox.EditStreamProc(IntPtr dwCookie, IntPtr buf, Int32 cb, Int32& transferred)
   at System.Windows.Forms.UnsafeNativeMethods.CallWindowProc(IntPtr wndProc, IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   at System.Windows.Forms.NativeWindow.DefWndProc(Message& m)
   at System.Windows.Forms.Control.DefWndProc(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.TextBoxBase.WndProc(Message& m)
   at System.Windows.Forms.RichTextBox.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

private async void FindAllBTN_Click(object sender, EventArgs e)
{
  progressBar.Minimum = 0;
  if (!isRunning)
  {
    isRunning = true;  <<============ I missed this line, Now it works smouthly
    OutputTB.Hide();
    OutputRTB.Show();
    FlipNextAllCKB.Checked = true;
    OutputRTB.Text = OutputTB.Text;

    FindAllBTN.Text = "Cancel";
    FindAllBTN.ForeColor = Color.Red;

    Progress<int> prog = new Progress<int>(SetProgress);
    m_cancelTokenSource = new CancellationTokenSource();

    try
    {
      await FindAllMatches(searchTB.Text, prog, m_cancelTokenSource.Token);
    }
    catch (OperationCanceledException)
    {
      //
    }
    finally
    {
      FindAllBTN.Text = "Find All";
      FindAllBTN.ForeColor = Color.Black;
      isRunning = false;
      m_cancelTokenSource = null;
      progressBar.Value = 0;
      replaceLTB.Focus();
    }
  }
  else
  {
    m_cancelTokenSource.Cancel();
  }

  return;

}

Quote:

This event is only a notification. Handling this event does not handle the exception or affect subsequent exception handling in any way. After the event has been raised and event handlers have been invoked, the common language runtime (CLR) begins to search for a handler for the exception. FirstChanceException provides the application domain with a first chance to examine any managed exception.