Null Exception when parsing HTML to String c#

private void createPDF()
{
    openFileDialog.Filter = "HTML Files (.html)|*.html";
    openFileDialog.FilterIndex = 0;
    if (openFileDialog.ShowDialog() == DialogResult.OK)
    {
        System.Text.StringBuilder store = new System.Text.StringBuilder();
        string fileName = openFileDialog.FileName;

        try
        {
            using (System.IO.StreamReader htmlReader = new System.IO.StreamReader(openFileDialog.FileName))
            {
                string line;
                while ((line = htmlReader.ReadLine()) != null)
                {
                    store.Append(line + Environment.NewLine);
                }
                this.textBox1.Text = store.ToString();
                string html = store.ToString();
                Document document = new Document(PageSize.LETTER, 20, 25, 35, 35);


                PdfWriter.GetInstance(document, new FileStream("c:\\Data/my.pdf", FileMode.Create));
                document.Open();
                System.Collections.Generic.List<IElement> htmlarraylist = new List<IElement>(HTMLWorker.ParseToList(new StringReader(html), new StyleSheet()));
                foreach (IElement element in htmlarraylist)
                {
                    document.Add(element);
                }
                document.Close();
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.ToString());
        }