Open-XML-SDK: ToFlatOpcString fails with generated document

Description

ToFlatOpcString throws “Root element is missing.” I’m trying to put this format into a VSTO add-in. When I enumerate wordprocessingDocument.Package.GetParts(), I get the relationship part back as an empty string (ContentType = "application/vnd.openxmlformats-package.relationships+xml"). Shouldn’t AddMainDocumentPart take care of this under the covers?

Information

• .NET Target: .NET Core 2.0, .NET Framework 4.7
• DocumentFormat.OpenXml Version: 2.8.1

Repro

using System;
using System.IO;
using DocumentFormat.OpenXml;
using DocumentFormat.OpenXml.Packaging;
using DocumentFormat.OpenXml.Wordprocessing;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            using (MemoryStream stream = new MemoryStream())
            using (WordprocessingDocument wordprocessingDocument = WordprocessingDocument.Create(stream, WordprocessingDocumentType.Document))
            {
                MainDocumentPart mainDocumentPart = wordprocessingDocument.AddMainDocumentPart();
                Body body = new Body(new Paragraph(new Run(new RunProperties(new Bold()),
                    new Text("Hello world this should be bold"))));
                mainDocumentPart.Document = new Document(body);

                wordprocessingDocument.Save();

                Console.WriteLine(wordprocessingDocument.ToFlatOpcDocument());
            }

        }
    }
}

Observed

Exception:

System.Xml.XmlException: Root element is missing.
   at System.Xml.XmlTextReaderImpl.Throw(Exception e)
   at System.Xml.XmlTextReaderImpl.ParseDocumentContent()
   at System.Xml.XmlTextReaderImpl.Read()
   at System.Xml.XmlReader.MoveToContent()
   at System.Xml.Linq.XElement.Load(XmlReader reader, LoadOptions options)
   at DocumentFormat.OpenXml.Packaging.OpenXmlPackage.GetContentsAsXml(PackagePart part)
   at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()
   at System.Xml.Linq.XContainer.AddContentSkipNotify(Object content)
   at System.Xml.Linq.XContainer.AddContentSkipNotify(Object content)
   at DocumentFormat.OpenXml.Packaging.OpenXmlPackage.ToFlatOpcDocument(XProcessingInstruction instruction)
   at ConsoleApp1.Program.Main(String[] args)

Expected

That ToFlatOpcString doesn’t throw and I get the resulting XML string.