线程“主”中的异常java.lang.NullPointerException。请帮助我

Question

我只是尝试从具有正确数据的Excel工作表中读取数据，然后使用迭代器将数据存储在数组列表中。代码没有错误。但它在运行时抛出错误。

public ArrayList<String> ReadExcelData(int ColNo) throws IOException 
    {
        FileInputStream fis = new FileInputStream("D:\\selenium\\GokulSheet2.xlsx");
        XSSFWorkbook Wb = new XSSFWorkbook(fis);    
        XSSFSheet s = Wb.getSheet("Sheet1");

        Iterator<Row> RowIt = s.iterator(); 
        RowIt.next();
         ArrayList<String> list = new ArrayList<String>();

        while(RowIt.hasNext())
        {
                 list.add(RowIt.next().getCell(ColNo).getStringCellValue());
        }
        //Wb.close();
        System.out.println("List::"+list);
        return list;
    }

    public static void main(String[]args) throws IOException, InterruptedException
    {
        POIXcell F = new POIXcell(); // POIXcell is class name
        F.ReadExcelData(0);
    }
}