Created Unassigned: Object reference not set to an instance of an object [8960]

Hi again,

Seems I've run into an issue with opening Excel documents generated by Microsoft's OpenXML. I receive an "Object reference not set to an instance of an object" error when trying to open the attached document.

If I open the document with Excel, and re-save it, it opens with ClosedXML fine. Just the original document refuses to open with ClosedXML.

I've included the Stack trace below, though it only really points to LoadSpreadsheetDocument.

```
Object reference not set to an instance of an object.

at ClosedXML.Excel.XLWorkbook.LoadSpreadsheetDocument(SpreadsheetDocument dSpreadsheet)
at ClosedXML.Excel.XLWorkbook.LoadSheets(String fileName)
at ClosedXML.Excel.XLWorkbook.Load(String file)
at ClosedXML.Excel.XLWorkbook..ctor(String file, XLEventTracking eventTracking)
at ClosedXML.Excel.XLWorkbook..ctor(String file)
at ClosedXML_Test.Form1.button1_Click(Object sender, EventArgs e) in D:\ClosedXML Test\Form1.cs:line 21
at System.Windows.Forms.Control.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnClick(EventArgs e)
at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ButtonBase.WndProc(Message& m)
at System.Windows.Forms.Button.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(Form mainForm)
at ClosedXML_Test.Program.Main() in D:\ClosedXML Test\Program.cs:line 18
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
```