NullReferenceException on XLWorkbook.Save()

[v0idname]

Do you want to request a feature or report a bug?

• Bug
• Feature

Version of ClosedXML

0.85.0, 0.86.0, 0.87.1, 0.88.0

What is the current behavior?

NullReferenceException on second call of XLWorkbook.Save() after XLWorksheet.Delete():

ClosedXML.Excel.XLWorkbook.DeleteSheetAndDependencies(WorkbookPart wbPart, String sheetId)
ClosedXML.Excel.XLWorkbook.<>c__DisplayClass94_0.<CreateParts>b__7(String ws)
System.Collections.Generic.List`1.ForEach(Action`1 action)
ClosedXML.Excel.XLWorkbook.CreateParts(SpreadsheetDocument document, Boolean evaluateFormulae)
ClosedXML.Excel.XLWorkbook.CreatePackage(String filePath, SpreadsheetDocumentType spreadsheetDocumentType, Boolean validate, Boolean evaluateFormulae)
ClosedXML.Excel.XLWorkbook.Save(Boolean validate, Boolean evaluateFormulae)
ClosedXML.Excel.XLWorkbook.Save()

• I attached a sample spreadsheet.
  test.xlsx

Code to reproduce problem:

using ClosedXML.Excel;
public void Main()
{
    string fileName = ".\\test.xlsx";

    // file can be created manually or by ClosedXML:
    XLWorkbook book1 = new XLWorkbook();
    book1.AddWorksheet("sheet1");
    book1.AddWorksheet("sheet2");
    book1.SaveAs(fileName);

    XLWorkbook book2 = new XLWorkbook(fileName);
    book2.Worksheet(1).Delete();
    book2.Save();
    book2.Save();    // <- NullReferenceException
}

[igitur]

Issue template clearly states: Attach the file.

[v0idname]

I updated issue.
I didn't attach the file earlier, because I have an error with any file (created manually or by ClosedXML, empty or not).

[igitur]

ClosedXML v0.89.0-beta1 has been published and is ready for your testing.