Java - GUI Query, all buttons are quitting the program?

Question

I'm just working through an online course and I have replicated a GUI program up to this point, two text fields and six buttons. We are at a point where we are assigning event responses to each button. I have assigned responses to both the Clear button and the Quit button but now all buttons are quitting the program and I'm not sure why. Can anyone help?

import javax.swing.JFrame;

public class CalcTest 
{
    public static void main(String[] args) 
    {
        Calc application = new Calc();
        application.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);//ensures program ends when you close the jframe
        application.setSize(400 , 400);
        application.setVisible(true);
    }
}

Second

import java.awt.Color;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.JButton;
import javax.swing.JPanel;
import java.awt.GridLayout;
import java.awt.event.ActionListener;//needed for detecting when a user has pressed Enter
import java.awt.event.ActionEvent;


public class Calc extends JFrame
{
    private JLabel firstNumberLabel; //use to display text that a user cannot edit
    private JTextField firstNumberField;
    
    private JLabel secondNumberLabel; 
    private JTextField secondNumberField;
    
    private JTextField resultField;
    private JPanel buttonPanel = new JPanel();//example of button panel being created outside of constructor
    private JButton addButton = new JButton("Add");
    private JButton subtractButton = new JButton("Subtract");
    private JButton multiplyButton = new JButton("Multiply");
    private JButton divideButton = new JButton("Divide");
    private JButton clearButton = new JButton("Clear");
    private JButton quitButton = new JButton("Quit");
    
    public Calc()
    {
        setLayout(null);//turn off Layout Manager to manually change layout
        firstNumberLabel = new JLabel("Enter First Number: ");//Label used to display text that cannot be edited
        firstNumberLabel.setBounds(10, 10, 150, 25);//set location in the frame
        add(firstNumberLabel);//this "add" method is inherited from class JFrame
        
        firstNumberField = new JTextField("0.00");//Field used to display text that can be edited 
        firstNumberField.setBounds(170 , 10 , 150 , 25 );
        add(firstNumberField);
        
        secondNumberLabel = new JLabel("Enter Second Number: ");
        secondNumberLabel.setBounds(10, 45 , 150, 25);//set location in the frame
        add(secondNumberLabel);
        
        secondNumberField = new JTextField("0.00");
        secondNumberField.setBounds(170 , 45 , 150 , 25 );
        add(secondNumberField);
        
        resultField = new JTextField("0.00");//result of calculation box in centre of frame
        resultField.setBounds(100 , 90 , 200 , 50);
        resultField.setEditable(false);//ensure the user cannot edit this box
        add(resultField);
        
        buttonPanel.setBounds(100 , 160 , 200 , 150);
        buttonPanel.setBackground(Color.red); //set colour to make it stand out from the grey
        add(buttonPanel);//"add" adds to the JFrame
        
        buttonPanel.setLayout(new GridLayout (3,2));//GridLayout automatically sorts in rows and columns
        JButton[] buttons = {addButton , subtractButton , multiplyButton , 
                            divideButton , clearButton , quitButton};
        ButtonHandler handler = new ButtonHandler();
        
        for(JButton button : buttons)
        {    
            buttonPanel.add(button);
            button.addActionListener(handler);
        }//end for
     }//end of constructor for class Calc 
    
    private class ButtonHandler implements ActionListener
    {
       public void actionPerformed(ActionEvent event) 
       {
                if(event.getSource() == clearButton)
                {
                    firstNumberField.setText("0.00");
                    secondNumberField.setText("0.00");

                }
                else if(event.getSource() == quitButton);
                {
                    System.exit(0);
                }
       }
    }//end of inner class ButtonHandler
        
}//end class Calc