Problem in adding file names to .txt file

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ADO.NET

C#3.5

I,m getting error(object reference not set to an instance of an object) when i try to add file names to list

button1_click--- for getting the folder path to textbox1
button2_click--- for storing filenames to list
button3_click--- for saving filenames to report.txt

public partial class Form1 : Form
{
    List<string> fileStore = null;
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        FolderBrowserDialog folderBrowserDlg = new FolderBrowserDialog();
    
        folderBrowserDlg.ShowNewFolderButton = true;
      
        DialogResult dlgResult = folderBrowserDlg.ShowDialog();
        if (dlgResult.Equals(DialogResult.OK))
        {
        
            textBox1.Text = folderBrowserDlg.SelectedPath;
           
            Environment.SpecialFolder rootFolder = folderBrowserDlg.RootFolder;
        }
    }

    private void button2_Click(object sender, EventArgs e)
    {
         DirectoryInfo dir1 = new DirectoryInfo(textbox1.text);
        FileInfo[] files = dir1.GetFiles("*.*", SearchOption.AllDirectories);
        
        try
        {
            foreach (FileInfo f in files)
            {

                fileStore.Add(f.Name.ToString());

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

    private void button3_Click(object sender, EventArgs e)
    {
        const string sPath = "report.txt";

        System.IO.StreamWriter SaveFile = new System.IO.StreamWriter(sPath);
      
        foreach (string item in fileStore)
        {
             SaveFile.WriteLine(item.ToString());
        }
        
        SaveFile.Close();

        MessageBox.Show("Programs saved!");
    }
}

[Edit member="Tadit"]
Urgency removed.
Corrected formatting and/or grammatical issues.
[/Edit]