PDFsharp 1.50测试版3:将密码添加到PDF时出现空所有者密码错误

我正在探索PDFsharp库，遇到了密码保护PDF的问题。

在他们的网站上以这个例子为例http://www.pdfsharp.com/PDFsharp/index.php?option=com_content&任务=视图&id＝36&项目ID=47，

这是我的代码

 try
 {
   string filename = "hi.pdf";
   File.Copy(Path.Combine("C:/User/Ichigo/Desktop", filename),
   Path.Combine(Directory.GetCurrentDirectory(), filename), true);
   PdfDocument document = PdfReader.Open(filename, "some text");
   PdfSecuritySettings securitySettings = document.SecuritySettings;
   securitySettings.UserPassword="user";
   securitySettings.OwnerPassword="owner";
   securitySettings.PermitAccessibilityExtractContent = false;
   securitySettings.PermitAnnotations = false;
   securitySettings.PermitAssembleDocument = false;
   securitySettings.PermitExtractContent = false;
   securitySettings.PermitFormsFill = true;
   securitySettings.PermitFullQualityPrint = false;
   securitySettings.PermitModifyDocument = true;
   securitySettings.PermitPrint = false;
   document.Save(filename);
 }
 catch (Exception e) 
 { throw new Exception("Something went wrong : " + e); }

每当我添加设置密码的代码时：

securitySettings.UserPassword="user";
securitySettings.OwnerPassword="owner";

当我到达保存PDF 的行时，我得到System.NullReferenceException: Object reference not set to an instance of an object.

document.Save（文件名）；我也收到错误信息

---- DEBUG ASSERTION FAILED ----
---- Assert Short Message ----
Empty owner password.
---- Assert Long Message ----
   at PdfSharp.Pdf.Security.PdfStandardSecurityHandler.PrepareEncryption() in c:'Users'Ichigo'Downloads'PDFsharp-MigraDocFoundation-1_50-beta3b'PDFsharp'src'PdfSharp'Pdf.Security'PdfStandardSecurityHandler.cs:line 590
   at PdfSharp.Pdf.PdfDocument.DoSave(PdfWriter writer) in c:'Users'Ichigo'Downloads'PDFsharp-MigraDocFoundation-1_50-beta3b'PDFsharp'src'PdfSharp'Pdf'PdfDocument.cs:line 392
   at PdfSharp.Pdf.PdfDocument.Save(Stream stream, Boolean closeStream) in c:'Users'Ichigo'Downloads'PDFsharp-MigraDocFoundation-1_50-beta3b'PDFsharp'src'PdfSharp'Pdf'PdfDocument.cs:line 325
   at PdfSharp.Pdf.PdfDocument.Save(Stream stream) in c:'Users'Ichigo'Downloads'PDFsharp-MigraDocFoundation-1_50-beta3b'PDFsharp'src'PdfSharp'Pdf'PdfDocument.cs:line 352
   at PdfSharp.Pdf.PdfDocument.Save(String path) in c:'Users'Ichigo'Downloads'PDFsharp-MigraDocFoundation-1_50-beta3b'PDFsharp'src'PdfSharp'Pdf'PdfDocument.cs:line 258

为什么我会出现这些错误？

编辑-这是System.NullReferenceException. 的堆栈跟踪

Something went wrong : `System.NullReferenceException: Object reference not set to an instance of an object.`     
 at PdfSharp.Pdf.Security.PdfStandardSecurityHandler.SetHashKey(PdfObjectID id)
   at PdfSharp.Pdf.IO.PdfWriter.WriteBeginObject(PdfObject obj)
   at PdfSharp.Pdf.PdfDictionary.WriteObject(PdfWriter writer)
   at PdfSharp.Pdf.PdfPage.WriteObject(PdfWriter writer)
   at PdfSharp.Pdf.PdfDocument.DoSave(PdfWriter writer)
   at PdfSharp.Pdf.PdfDocument.Save(Stream stream, Boolean closeStream)
   at PdfSharp.Pdf.PdfDocument.Save(Stream stream)
   at PdfSharp.Pdf.PdfDocument.Save(String path)

// HACK: Remove XRefTrailer
if (_trailer is PdfCrossReferenceStream)
{
    // HACK^2: Preserve the SecurityHandler.
    PdfStandardSecurityHandler securityHandler = _securitySettings.SecurityHandler;
    _trailer = new PdfTrailer((PdfCrossReferenceStream) _trailer);
    _trailer._securityHandler = securityHandler;
}