I’m coding GUI for my Java project. Here are some info:

1. I have an interface EmployeeController.

public interface EmployeeController {

    void addEmployee(Employee employee);

    Employee getEmployeeById(int employeeId);

    void updateEmployee(Employee employee);

    void deleteEmployee(int employeeId);

    List<Employee> getAllEmployees();
}

2. I have EmployeeControllerImpl implements EmployeeController, the methods are coded here.
3. Then I code my JFrame AddEmploye where I want to add employee details into text fields and a button to save them.

public class AddEmployee extends javax.swing.JFrame {

    private EmployeeController employeeController;

    // Constructor to accept Controller
    public AddEmployee(EmployeeController employeeController) {
        this.employeeController = employeeController;
        initComponents();
        addActionListeners();
    }

    // Method to add action listeners to components
    private void addActionListeners() {
        saveButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
                System.out.println("Save button clicked");
                saveButtonActionPerformed(evt);
            }
        });
    }
    // Button method
    private void saveButtonActionPerformed(java.awt.event.ActionEvent evt) {                                           
        Employee employee = new Employee();
        employee.setName(nameText.getText());
        employee.setAddress(addressText.getText());
        employee.setPhoneNumber(phoneNumberText.getText());
        try {
            employee.setAge(Integer.parseInt(ageText.getText()));
            employee.setSalary(Double.parseDouble(salaryText.getText()));
        } catch (NumberFormatException e) {
            JOptionPane.showMessageDialog(this, "Invalid input format.");
            return;
        }
        employee.setGender(genderText.getText());
        employee.setPosition(positionText.getText());

        employeeController.addEmployee(employee);
        JOptionPane.showMessageDialog(this, "Employee added successfully!");
    }
    
    // Rest of the pre-generated code by Swing
}

So when I run the frame and click on Save button. The program seemingly stopped at constructor with:
Cannot invoke “EmployeeController.addEmployee(Employee)” because “this.employeeController” is null.

I have been looking into this problem for a while but still clueless.