A defect in the updated NPOI Wrapper

I've discovered a defect in the new NPOI wrapper I built, where creating a XLSX file results in Excel saying there was "unreadable content," and that some data corruption occurred. I think I've fixed the code, but will need several more days to do some exhaustive testing. The exact same code with XLS format does not result in the error - it's only using the XSSFWorkbook that this error occurs, which leads me to believe that the NPOI 2 beta is not as stable as I thought...although, it still is a great tool, despite this.