iTextSharp错误文档在打开之前没有页面

我有一个问题，我想在我的pdf文档中添加一个页眉和页脚。为此，我使用了PdfPageEventHelper。

我创建了一个类：

    public class PDFFooter : PdfPageEventHelper
    {
        // write on top of document
        public override void OnOpenDocument(PdfWriter writer, Document document)
        {
            base.OnOpenDocument(writer, document);
        }
        // write on start of each page
        public override void OnStartPage(PdfWriter writer, Document document)
        {
            base.OnStartPage(writer, document);
            EnTeteDePage(document);
        }
        // write on end of each page
        public override void OnEndPage(PdfWriter writer, Document document)
        {
            base.OnEndPage(writer, document);
            PiedDePage(document);
        }
        //write on close of document
        public override void OnCloseDocument(PdfWriter writer, Document document)
        {
            base.OnCloseDocument(writer, document);
        }
        private void EnTeteDePage(Document document)
        {
            Controleur.ControleurParametre CP = new ControleurParametre();
            T_PARAMETRE _param = CP.Charger();
            T_ADRESSE _adresseParam = CP.ChargerAdresse(_param.ID_ADRESSE_SOCIETE);
            iTextSharp.text.Image img = (_param.PATH_LOGO_SOCIETE != "" && _param.PATH_LOGO_SOCIETE != null) ? iTextSharp.text.Image.GetInstance(_param.PATH_LOGO_SOCIETE) : null;
            if (img != null)
            {
                if (img.Width / 150 > img.Height / 150) img.ScalePercent((img.Width / 150) * 100);
                else img.ScalePercent((img.Height / 150) * 100);
                document.Add(img);
            }
            PdfPTable head = new PdfPTable(6);
            PdfPCell cell1 = new PdfPCell(img);
            PdfPCell cell2 = new PdfPCell(new Phrase("Rapport de réception de chantier 'n" + _param.NOM_SOCIETE + " - " +
                _adresseParam.ADDR_VOIE + " - " +
                _adresseParam.ADDR_VOIE_BIS + "'n" +
                _adresseParam.ADDR_CP + " - " +
                _adresseParam.ADDR_VILLE + "'n"));
            float[] wid = new float[] { 0.1f, 0.9f };
            head.SetWidths(wid);
            cell1.HorizontalAlignment = 1;
            cell2.HorizontalAlignment = 1;
            head.AddCell(cell1);
            head.AddCell(cell2);
            document.Add(head);
        }
        private void PiedDePage(Document document)
        {
            Paragraph Footer = new Paragraph();
            document.Add(Footer);
        }
    }

我使用一种方法打开我的文档：

    private async Task<bool> ouvrirPDF(iTextSharp.text.Rectangle re)
    {
        str = await f.OpenAsync(Windows.Storage.FileAccessMode.ReadWrite);
        st = str.AsStreamForWrite();
        using (myDocument = new Document(re)) //Création du PDF
        {
            using (writer = PdfWriter.GetInstance(myDocument, st))
            {
                PDFFooter foot = new PDFFooter();
                writer.PageEvent = foot;
                myDocument.Open(); //Ouverture du PDF
                cb = writer.DirectContent;
            }
        }
        return true;
    }

但是当我尝试执行myDocument.Open（）时，我有一个异常"文档没有页面"

你能帮我解决吗？