pdfReader抛出System.NullReferenceException错误

Question

 private void highlightPDFAnnotation(string outputFile, int pageno, string[] splitText)
{
    try
    {
        PdfReader reader = new PdfReader(System.IO.File.ReadAllBytes(outputFile));
        using (FileStream fs = new FileStream(outputFile, FileMode.Open, FileAccess.Write, FileShare.None))
        {
            using (PdfStamper stamper = new PdfStamper(reader, fs))
            {
                myLocationTextExtractionStrategy strategy = new myLocationTextExtractionStrategy();
                string currentText = PdfTextExtractor.GetTextFromPage(reader, pageno, strategy);
                for (int i = 0; i < splitText.Length; i++)
                {
                    List<iTextSharp.text.Rectangle> MatchesFound = strategy.GetTextLocations(splitText[i].Trim(), StringComparison.CurrentCultureIgnoreCase);
                    foreach (Rectangle rect in MatchesFound)
                    {
                        float[] quad = { rect.Left - 3.0f, rect.Bottom, rect.Right, rect.Bottom, rect.Left - 3.0f, rect.Top + 1.0f, rect.Right, rect.Top + 1.0f };
                        //Create our hightlight
                        PdfAnnotation highlight = PdfAnnotation.CreateMarkup(stamper.Writer, rect, null, PdfAnnotation.MARKUP_HIGHLIGHT, quad);
                        //Set the color
                        highlight.Color = BaseColor.YELLOW;

                        PdfAppearance appearance = PdfAppearance.CreateAppearance(stamper.Writer, rect.Width, rect.Height);
                        PdfGState state = new PdfGState();
                        state.BlendMode = new PdfName("Multiply");
                        appearance.SetGState(state);
                        appearance.Rectangle(0, 0, rect.Width, rect.Height);
                        appearance.SetColorFill(BaseColor.YELLOW);
                        appearance.Fill();

                        highlight.SetAppearance(PdfAnnotation.APPEARANCE_NORMAL, appearance);

                        //Add the annotation
                        stamper.AddAnnotation(highlight, pageno);                            

                        highlight.Clear();

                    }
                }
                stamper.Close();
                stamper.Dispose();

            }
            fs.Close();
            fs.Dispose();


        }
        reader.Close();

        reader = null;
    }
    catch (Exception ex)
    {
        throw;
    }
}

第四次尝试后，它会抛出＆＃34;对象引用未设置为对象的实例。＆＃34; 错误。

Stacktrace：

  

at iTextSharp.text.pdf.PdfReader.PageRefs.ReadPages（）
  在iTextSharp.text.pdf.PdfReader.PageRefs..ctor（PdfReader阅读器）
  在iTextSharp.text.pdf.PdfReader.ReadPages（）
  在iTextSharp.text.pdf.PdfReader.ReadPdf（）
  在iTextSharp.text.pdf.PdfReader..ctor（Byte [] pdfIn，Byte []   ownerPassword）在iTextSharp.text.pdf.PdfReader..ctor（Byte [] pdfIn）   at bottom.highlightPDFAnnotation（String outputFile，Int32 pageno，   String [] splitText）