Why does defaultTableModel undergo nullPointerException?

Question

Why my defaultTableModel undergo nullPointerException?

 private void jYearChooser1PropertyChange(java.beans.PropertyChangeEvent evt) {
        try {
            dtm.setRowCount(0);//NullPointerException
            ArrayList<Salary> salaries = SalaryController.getSalaries(jYearChooser1.getYear(), jMonthChooser1.getMonth() + 1);
            for (Salary salary : salaries) {
                Employee employee = EmployeeController.searchEmployee(salary.getEmployeeId());
                Object[] rawData = {salary.getEmployeeId(), employee.getFirstName() + " " + employee.getSecondName(), salary.getTotalEarnings(), salary.getTotalDeductions(), salary.getNetSalary()};
                dtm.addRow(rawData);
            }
        } catch (ClassNotFoundException | SQLException ex) {
            Logger.getLogger(ViewSalary.class.getName()).log(Level.SEVERE, null, ex);
        }

    }