[Solved] ITextSharp 5.3 PdfCopy Error: Object reference not set to an instance of an object

getting error System.NullReferenceException: 'Object reference not set to an instance of an object.' on this line

using (PdfCopy copy = new PdfCopy(docu, new FileStream(outputFilePath, FileMode.Create)))

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using Apitron.PDF.Rasterizer;
using Apitron.PDF.Rasterizer.Configuration;
using iTextSharp.text;
using iTextSharp.text.pdf;

namespace SampleWinform
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            this.WindowState = FormWindowState.Maximized;
            this.AutoSize = true;
        }
        private List<int> selectedPageNumbers = new List<int>();//list to store the selected page numbers.
        OpenFileDialog Opd = new OpenFileDialog();
        FileStream fs;
        private void PdfUpload_Click(object sender, EventArgs e)
        {

            Opd.Filter = "(*.pdf)|*.pdf";
            Opd.Title = "Select PDF File";
            if (Opd.ShowDialog() == DialogResult.OK)
            {
                try
                {
                    fs = new FileStream(Opd.FileName, FileMode.Open);
                    pdfViewer1.Document = new Apitron.PDF.Rasterizer.Document(fs);
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
            }
        }

        private void Break_Click(object sender, EventArgs e)
        {
            string currentPage = pdfViewer1.Document.Navigator.CurrentPage.Label;// Get the current page number
            selectedPageNumbers.Add(int.Parse(currentPage)); // Add the page number to the list
            UpdateTextBox();
        }
        private void UpdateTextBox()
        {
            // Update the textbox with the selected page numbers
            if (txtInput.Text == "")
            {
                txtInput.Text = string.Join(",", selectedPageNumbers);
            }
            else
            {
                txtInput.Text = txtInput.Text + "-" + (int.Parse(pdfViewer1.Document.Navigator.CurrentPage.Label) - 1).ToString() + "," + pdfViewer1.Document.Navigator.CurrentPage.Label;
                //string.Join(",", selectedPageNumbers);
            }
        }
        private void SplitButton_Click(object sender, EventArgs e)
        {
            //if (int.Parse(pdfViewer1.Document.Navigator.ToString()) == 0)
            //{
            // MessageBox.Show("Please load a PDF file first.");
            // return;
            //}

            string pageRangesText = txtInput.Text;
            string[] range = pageRangesText.Split(new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
            List<Tuple<int, int>> pageRanges = new List<Tuple<int, int>>();

            foreach (string rangePair in range)
            {
                string outputDirectory = "C://SplitFiles";// Specify the path where you want to save the split PDFs
                fs.Position = 0;
                using (PdfReader reader = new PdfReader(fs))
                {
                    int pageCount = reader.NumberOfPages;
                    using (iTextSharp.text.Document docu = new iTextSharp.text.Document(PageSize.A4))
                    {
                        foreach (var ranges in range)
                        {
                            string[] rangePairs = rangePair.Trim().Split(new char[] { '-' }, StringSplitOptions.RemoveEmptyEntries);
                            if (rangePairs.Length == 2)
                            {
                                int startPage = int.Parse(rangePairs[0]);
                                int endPage = int.Parse(rangePairs[1]);

                                if (startPage <= endPage)
                                {
                                    string outputFileName = $"PdfPages{startPage}-{endPage}.pdf";
                                    string outputFilePath = Path.Combine(outputDirectory, outputFileName);
                                    using (PdfCopy copy = new PdfCopy(docu, new FileStream(outputFilePath, FileMode.Create)))
                                    {
                                        docu.Open();
                                        for (int page = startPage; page <= endPage; page++)
                                        {
                                            PdfImportedPage importedPage = copy.GetImportedPage(reader, page);
                                            copy.AddPage(importedPage);
                                        }
                                    }
                                }
                            }
                            else if (rangePairs.Length == 1)
                            {
                                int startPage = int.Parse(rangePairs[0]);
                                int endPage = pageCount;

                                if (startPage <= endPage)
                                {
                                    string outputFileName = $"PdfPages{startPage}-{endPage}.pdf";
                                    string outputFilePath = Path.Combine(outputDirectory, outputFileName);

                                    using (PdfCopy copy = new PdfCopy(docu, new FileStream(outputFilePath, FileMode.Create)))
                                    {
                                        docu.Open();
                                        for (int page = startPage; page <= endPage; page++)
                                        {
                                            PdfImportedPage importedPage = copy.GetImportedPage(reader, page);
                                            copy.AddPage(importedPage);
                                        }
                                    }
                                }
                            }
                        }
                        MessageBox.Show($"PDF has Splitted Completely.");
                    }
                }
            }

            if (pageRanges.Count == 0)
            {
                MessageBox.Show("Invalid page ranges format.");
                return;
            }
        }
    }
}

why there is getting error

please Help Me team