ZickZack has uploaded a patch.
Description:
I have found an Excel Sheet where "WorkbookProperties" are null. And this throws an Exception in private void LoadSpreadsheetDocument(SpreadsheetDocument dSpreadsheet) but Excel 2007 can work without any problem with the same Sheet.
So I think this change will help, please look for the
Remark // dm 20130422 WorkbookProperties might be null