C# Save to FILE.PDF

Question

I got error when i try to match PdfWritter to PdfWriter.GetInstance pdfDoc and memoryStream.

A exceção é "System.NullReferenceException: Referência do objeto não definida como uma instância de um objeto."

 private async void SaveToPDF(object sender, RoutedEventArgs e)
    {
        StringBuilder sb = new StringBuilder();
        sb.Append("Hi");
       
        StringReader sr = new StringReader(sb.ToString());

        Document pdfDoc = new Document(PageSize.A4, 10f, 10f, 10f, 0f);

        HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
        using (MemoryStream memoryStream = new MemoryStream())
        {
            //error comes here
            PdfWriter writer = PdfWriter.GetInstance(pdfDoc, memoryStream);
            pdfDoc.Open();

            htmlparser.Parse(sr);
            //error comes here
            pdfDoc.Close();

            byte[] bytes = memoryStream.ToArray();
            File.WriteAllBytes($"test.PDF", bytes);

            memoryStream.Close();
        }

    }