XML to CSV, getting error

[saldous]

Hi, looking for some troubleshooting assistance in the below code please. I keep getting the error: NullReferenceException was unhandled: Object reference not set to an instance of an object

Been scratching my head for days, any ideas? Thanks!
(I've attached the sample XML file - saved as .txt so I could upload)

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Xml;
using System.IO;

namespace XML_to_CSV_test2
{
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }

    // N th lever recursion subfunction
    private void getsubnode(XmlNode vnode, StreamWriter outfilewriter)
    {
      while (vnode.HasChildNodes == true)
      {
        string stringtest = "";
        foreach (XmlAttribute atttest1 in vnode.Attributes)
        {
          stringtest = stringtest + atttest1.Value + ",";
        }

        outfilewriter.WriteLine(stringtest);

        foreach (XmlNode vchildnode in vnode)
        {
          getsubnode(vchildnode, outfilewriter);
        }
        return;
      }

      while (vnode.HasChildNodes == false)
      {
        string stringtest = "";
        foreach (XmlAttribute atttest1 in vnode.Attributes)
        {
          stringtest = stringtest + atttest1.Value + ",";
        }
        outfilewriter.WriteLine(stringtest);
        return;
      }
    }


    // Browse dialog button
    private void btnBrowse_Click(object sender, EventArgs e)
    {
      openFileDialog1.ShowDialog();
      txtBoxSourceFilePath.Text = openFileDialog1.FileName;  
    }

    // Convert button
    private void btnConvert_Click(object sender, EventArgs e)
    {
      XmlDocument test = new XmlDocument();
      XmlNode test2;
      StreamWriter outwriter;
      FileStream outfile = new FileStream(txtBoxSourceFilePath.Text + ".csv", FileMode.OpenOrCreate);

      test.Load(txtBoxSourceFilePath.Text);

      test2 = test.DocumentElement;

      outwriter = new StreamWriter(outfile);

      getsubnode(test2, outwriter);

      outwriter.Close();
      outfile.Close(); 

    }
  }
}