Since paymentMilesrtone is not mandatory field, so add null checking when reading corressponding cell value

src/main/java/com/ffii/tsms/modules/project/service/InvoiceService.kt

@@ -404,7 +404,7 @@ open class InvoiceService(
                 team = ExcelUtils.getCell(sheet, i, 3).stringCellValue
                 stage = ExcelUtils.getCell(sheet, i, 4).numericCellValue.toString()
                 paymentMilestone = ExcelUtils.getCell(sheet, i, 5).stringCellValue
-                paymentMilestoneDate = ExcelUtils.getCell(sheet, i, 6).dateCellValue.toInstant().atZone(ZoneId.systemDefault()).toLocalDate()
+                paymentMilestoneDate = ExcelUtils.getCell(sheet, i, 6).dateCellValue?.toInstant()?.atZone(ZoneId.systemDefault())?.toLocalDate()
                 client = ExcelUtils.getCell(sheet, i, 7).stringCellValue
                 address = ExcelUtils.getCell(sheet, i, 8).stringCellValue
                 attention = ExcelUtils.getCell(sheet, i, 9).stringCellValue