Pdf generation 生成PDF文档时出现GemBox错误

我在应用程序中使用GemBox.Document生成PDF版本的搜索结果时遇到了一个异常问题。当我返回了大量的记录时，这种情况似乎会发生，尽管数量仍然很小。奇怪的是，如果我为所有记录的子集a和所有记录的子集B生成一个报告，而不是同时为a和B生成一个报告，那么代码就会起作用。在这三种情况下，DocumentModel似乎都能正确生成；保存时会出现错误

以下是相关方法：

public MemoryStream GetPdfReport(SearchRequestData searchRequestData, HttpRequest httpRequest)
        {
            _stream = new MemoryStream();
            _httpRequest = httpRequest;
            DocumentModel dm = CreateDocxOrPdfReport(searchRequestData);
            dm.Save(_stream, GemBox.Document.SaveOptions.PdfDefault);
            //CreateDocxOrPdfReport(searchRequestData).Save(_stream, GemBox.Document.SaveOptions.PdfDefault);

            return _stream;
        }

private DocumentModel CreateDocxOrPdfReport(SearchRequestData searchRequestData)
        {
            SetFieldsToDisplay(searchRequestData);
            searchRequestData.PageIndex = 0;

            var reportData = TrackService.GetTrackReportData(searchRequestData);
            var doc = new DocumentModel();
            var section = new Section(doc);
            doc.Sections.Add(section);
            AddDocxOrPdfHeaderAndFooter(doc, "Search Report");
            AddDocxOrPdfFirstPageSummary(doc, "Search Report", reportData.Count, searchRequestData, ReportType.Track);
            AddDocxOrPdfReportData(doc, doc.Sections[0], reportData);
            return doc;
        }

下面是错误：

System.NullReferenceException was unhandled by user code
  HResult=-2147467261
  Message=Object reference not set to an instance of an object.
  Source=GemBox.Document
  StackTrace:
       at eh.k()
       at eh.a(f6 A_0, ko A_1)
       at b9.a(f6& A_0, Int32 A_1, gl A_2, v3 A_3, Int32& A_4, Boolean& A_5, ArrayList& A_6)
       at b9.a(gl A_0, f6& A_1, Int32& A_2)
       at b9.a(l4 A_0, gl A_1)
       at ug.a(tz A_0, e6 A_1, gl A_2)
       at pc.a(er A_0, er A_1, Double A_2, t1 A_3, Boolean A_4, ug A_5)
       at pc.a(t1 A_0, t1 A_1, t1 A_2, Boolean A_3, ug A_4, d2 A_5, d2 A_6)
       at pc.a(ir A_0, Double A_1, Boolean A_2, ug A_3)
       at pc.a(x9 A_0, f6 A_1, ir A_2, ug A_3)
       at pc.a(f6 A_0, ko A_1)
       at b9.a(f6& A_0, Int32 A_1, gl A_2, v3 A_3, Int32& A_4, Boolean& A_5, ArrayList& A_6)
       at b9.a(gl A_0, f6& A_1, Int32& A_2)
       at b9.a(l4 A_0, gl A_1)
       at b9.a(l4 A_0)
       at qh.a(l4 A_0)
       at a3.e()
       at ai.a(Boolean A_0)
       at ai.g()
       at ai.c()
       at vt.a()
       at ow.a(Stream A_0, String A_1, String A_2, db A_3)
       at vt.a(vw A_0, Stream A_1, String A_2, String A_3, db A_4)
       at GemBox.Document.PdfSaveOptions.Save(DocumentModel document, Stream stream, String path)
       at GemBox.Document.DocumentModel.Save(Stream stream, SaveOptions options)
       at StateScape.API.Services.LocalTrackExportService.GetPdfReport(SearchRequestData searchRequestData, HttpRequest httpRequest) in c:\Users\Andrew\Documents\Projects\StateScape\git\statescapeweb\StateScape.API\Services\LocalTrackExportService.cs:line 27
       at StateScape.API.Controllers.ExportReportController.GetLocalTrackPdf() in c:\Users\Andrew\Documents\Projects\StateScape\git\statescapeweb\StateScape.API\Controllers\ExportReportController.cs:line 244
       at lambda_method(Closure , ControllerBase , Object[] )
       at System.Web.Mvc.ActionMethodDispatcher.Execute(ControllerBase controller, Object[] parameters)
       at System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters)
       at System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters)
       at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass42.<BeginInvokeSynchronousActionMethod>b__41()
       at System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass8`1.<BeginSynchronous>b__7(IAsyncResult _)
       at System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult`1.End()
       at System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult)
       at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass37.<>c__DisplayClass39.<BeginInvokeActionMethodWithFilters>b__33()
       at System.Web.Mvc.Async.AsyncControllerActionInvoker.<>c__DisplayClass4f.<InvokeActionMethodFilterAsynchronously>b__49()
  InnerException: 

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您好，您是否使用复制此问题？ 如果您这样做了，您是否有可能向我们发送一个小型VS测试项目以及复制问题的输入文件，以便我们可以对其进行调查？

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如果可能的话，您也可以随时查看此问题的更多详细信息。

解决方案是使用正则表达式检查不可打印的字符。我仍然不确定为什么这些字符只在组合结果集时出现，但这对我来说是有效的