C#：应该怎么赋值才行？

运行时候老是提示未将对象引用到设置到对象实例中，应该怎么处理才对？
DataTable dt = new DataTable( FilePath);
FileStream fs = new FileStream("", FileMode.Open, FileAccess.Read);

        NPOI.XSSF.UserModel.XSSFWorkbook book = new NPOI.XSSF.UserModel.XSSFWorkbook(fs);
        //是否找到表头
        bool isTitle = false;
        NPOI.SS.UserModel.ISheet sheet = book.GetSheetAt(0);
        for (int i = 0; i <= sheet.LastRowNum; i++)
        {
            NPOI.SS.UserModel.IRow row = sheet.GetRow(i);
            if (row.GetCell(0).ToString() == "INVOICENO")
            {
                //表头行
                isTitle = true;
                if (row == null) continue;
                int firstCellNum = row.FirstCellNum;
                int lastCellNum = row.LastCellNum;
                if (firstCellNum == lastCellNum) continue;

                for (int ji = firstCellNum; ji < lastCellNum; ji++)
                {
                    if (!dt.Columns.Contains(row.GetCell(ji).ToString()))
                    {
                        dt.Columns.Add(row.GetCell(ji).StringCellValue, typeof(string));
                    }
                }
            }

            if (isTitle)
            {
                //其它行
                DataRow newRow = dt.Rows.Add();
                for (int j = 0; j < 14; j++)
                {
                    newRow[j] = row.GetCell(j).ToString();
                }
            }
        }

for 循环前需要先判断下数据是不是有
if(sheet.LastRowNum>0){
for (int i = 0; i <= sheet.LastRowNum; i++){
...
}
}

if (row.GetCell(0).ToString() == "INVOICENO") 这个判断也有问题
用equles
if ( "INVOICENO".equles(row.GetCell(0).ToString()))