Steps to reproduce

1. load attached lastSheetActive.xlsx file with 2.6.0.FL.20140722-Eval (you can use activeTab.zul)
2. delete 2nd sheet
3. export it
4. open it with Excel 2007

Actual Result:

Excel shows a popup said that the file has unreadable content, but it can be recovered by Excel.

Expected Result:

no error message shows.

Workaround

reset active sheet before exporting

Book book = spreadsheet.getBook();
book.setActiveSheet(book.getSheetIndex(spreadsheet.getSelectedSheet()));