How to make exit button to make everything invisible?

Question

I wrote a java program that asks the user to enter the name of the student and the GPA. It contains 3 buttons:

1. save: to save name and GPA in 2 arrays.
2. clear: to clear text field.
3. exit: to make everything invisible and the show student name with max GPA, and number of students

the maximum number of students is 100 and GPA must be 4 or below and when it is above 4 or below 0, an invalid message should appear.

but unfortunately, the exit button is not working and it gives me the following error:

Exception in thread "AWT-EventQueue-1" java.lang.NumberFormatException: empty String at sun.misc.FloatingDecimal.readJavaFormatString(FloatingDecimal.java:1842) at sun.misc.FloatingDecimal.parseDouble(FloatingDecimal.java:110) at java.lang.Double.parseDouble(Double.java:538) at StudentGPA.actionPerformed(StudentGPA.java:67) at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)

when the text field is empty the program gives me an error, but when it is not empty it makes everything invisible

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class StudentGPA extends JApplet implements ActionListener{

    JTextField t1,t2;
    JLabel l1,l2,l3,l4,invalid,max;
    Container cp;
    JPanel p1,p2,p3,p4,p5;
    JButton saveB,clearB,exitB;
    public void init()
    {
        cp= getContentPane();
        cp.setLayout(new GridLayout(5,1));


        p1= new JPanel(new FlowLayout(FlowLayout.LEFT));
        l1= new JLabel("Applet");
        p1.add(l1);
        cp.add(p1);


        p2 = new JPanel(new FlowLayout(FlowLayout.LEFT));
        l2= new JLabel("Enter students name and GPA: ");
        p2.add(l2);
        cp.add(p2);


        p3 = new JPanel(new FlowLayout(FlowLayout.LEFT));
        l3= new JLabel("Name: ");
        t1 = new JTextField(20);
        p3.add(l3);
        p3.add(t1);
        cp.add(p3);


        p4 = new JPanel(new FlowLayout(FlowLayout.LEFT));
        l4= new JLabel("GPA: ");
        invalid = new JLabel("Invalid GPA!");
        invalid.setVisible(false);
        t2 = new JTextField(15);
        p4.add(l4);
        p4.add(t2);
        p4.add(invalid);
        cp.add(p4);


        p5 = new JPanel(new FlowLayout(FlowLayout.CENTER));
        saveB = new JButton("Save");
        saveB.addActionListener(this);
        clearB = new JButton("Clear");
        clearB.addActionListener(this);
        exitB = new JButton("Exit");
        exitB.addActionListener(this);
        p5.add(saveB);
        p5.add(clearB);
        p5.add(exitB);
        cp.add(p5);

    }
    public void actionPerformed(ActionEvent e) {

        String ac=e.getActionCommand();
        String[] nArray = new String[100];
        double[] gArray = new double[100];
        String name=String.valueOf(t1.getText());
        double gpa=Double.parseDouble(t2.getText());
        double maxGPA=0;
        String maxName=null;
        int counter=0;

        if(ac.equals("Save") && gpa>=0 && gpa<=4&&counter<100)
        {
            for(int i=0;i<100;i++)
            {
                nArray[i]=name;
                gArray[i]=gpa;
                t1.setText(null);
                t2.setText(null);
                invalid.setVisible(false);

                if(gpa>maxGPA)
                {
                    maxGPA=gpa;
                    maxName=name;
                }

                counter++;

            }
        }

        else
        {
            invalid.setVisible(true);
        }


        if(ac.equals("Clear"))
        {
            t1.setText(null);
            t2.setText(null);
            invalid.setVisible(false);
        }

        else if(ac.equals("Exit"))
        {
            this.setVisible(false);
            l2.setText(maxName+" scored the maximum GPA ("+maxGPA+")\namong a total of "+counter+" students");
            l2.setVisible(true);
        }

    }
}

I expect when I click the exit button the name of max GPA student and number of students