خطا عند التعامل مع ملفات الاكسل في جافا

لدي برنامج يقوم بقراءة ملف اكسيل ويرفع محتوياته الى قاعدة البيانات وهو يعمل بشكل جيد لكن في حال كانت خلية ما فارغة يظهر لي الخطأ التالي:

java.lang.NullPointerException

هذا هو الكود الذي يحصل فيه الخطأ:

public void upload() throws IOException{
    String host="localhost";
    String port="543";
    String db_name="postgr";
    String username="root";
    String password="root";
    int max = 1000;
    int min = 0;
    String dir="C:\\Users\\User\\Desktop\\book.xlsx";
    int batchSize = 20;
    Connection connection = null;
    try {
        long start = System.currentTimeMillis();
        FileInputStream inputStream = new FileInputStream(dir);
        Workbook workbook = new XSSFWorkbook(inputStream);
        Sheet firstSheet = workbook.getSheetAt(0);
        Iterator<Row> rowIterator = firstSheet.iterator();
        try {
            Class.forName("org.postgresql.Driver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        connection = DriverManager.getConnection("jdbc:postgresql://"+host+":"+port+"/"+db_name+"", ""+username+"", ""+password+"");
        connection.setAutoCommit(false);
        String sql = "INSERT INTO survey.tabledata (id,col1,col2,col3,col4,col5,col6,col7,col8,col9,col10,surveyid) VALUES (?,?,?,?,?,?,?,?,?,?,?,?)";
        PreparedStatement statement = connection.prepareStatement(sql);    
        int count = 0;
        while (rowIterator.hasNext()) {
            Row nextRow = rowIterator.next();
            Iterator<Cell> cellIterator = nextRow.cellIterator();
            while (cellIterator.hasNext()) {
                Cell nextCell = cellIterator.next();
                int columnIndex = nextCell.getColumnIndex();
                switch (columnIndex) {
                case 0:
                    int col1 = (int) nextCell.getNumericCellValue(); 
                    statement.setInt(1,col1);
                    break;
                case 1:
                    String col2 = nextCell.getStringCellValue();
                    statement.setString(2, col2);
                    break;
                case 2:
                    String col3 = nextCell.getStringCellValue();
                    statement.setString(3, col3);
                    break;
                case 3:
                    String col4 = nextCell.getStringCellValue();
                    statement.setString(4, col4);
                    break;
                case 4:
                    String col5 = nextCell.getStringCellValue();
                    statement.setString(5, col5);
                    break;
                case 5:
                    String col6 = nextCell.getStringCellValue();
                    statement.setString(6, col6);
                    break;
                case 6:
                    String col7 = nextCell.getStringCellValue();
                    statement.setString(7, col7);
                    break;
                case 7:
                    String col8 = nextCell.getStringCellValue();
                    statement.setString(8, col8);
                    break;
                case 8:
                    String col9 = nextCell.getStringCellValue();
                    statement.setString(9, col9);
                    break;

                case 9:
                    String col10 = nextCell.getStringCellValue();
                    statement.setString(10, col10);
                    break;
                case 10:
                    String col11 = nextCell.getStringCellValue();
                    statement.setString(11, col11);
                    break;
                case 11:
                    int col12 = (int) nextCell.getNumericCellValue();
                    statement.setInt(12,col12);
                    break;
                }
            }
            statement.addBatch();

            if (count % batchSize == 0) {
                statement.executeBatch();
            }              
        }
        statement.executeBatch();
        connection.commit();
        connection.close();
        long end = System.currentTimeMillis();
        System.out.printf("import done");

    } catch (IOException ex1) {
        System.out.println("Error");
        ex1.printStackTrace();
    } catch (SQLException ex2) {
        System.out.println("error");
        ex2.printStackTrace();
    }
}

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