Thread: NullPointerException with java.io using a gui

Hello, this summer has become boring, so I decided to learn programming. I've had no problems until now. I'm learning the java.io class, and decided to use it in a gui. In the program I'm trying to test a number to see if it's prime then save it to a file if it is the largest prime tested.
I get a null pointer exception. Any help would be appreciated.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at sun.misc.FloatingDecimal.readJavaFormatString(Floa tingDecimal.java:989)
at java.lang.Double.parseDouble(Double.java:510)
at primeGui.actionPerformed(primeGui.java:65)
at javax.swing.AbstractButton.fireActionPerformed(Abs tractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed (AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed (DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultB uttonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseRe leased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.jav a:6288)
at javax.swing.JComponent.processMouseEvent(JComponen t.java:3267)
at java.awt.Component.processEvent(Component.java:605 3)
at java.awt.Container.processEvent(Container.java:204 1)
at java.awt.Component.dispatchEventImpl(Component.jav a:4651)
at java.awt.Container.dispatchEventImpl(Container.jav a:2099)
at java.awt.Component.dispatchEvent(Component.java:44 81)
at java.awt.LightweightDispatcher.retargetMouseEvent( Container.java:4577)
at java.awt.LightweightDispatcher.processMouseEvent(C ontainer.java:4238)
at java.awt.LightweightDispatcher.dispatchEvent(Conta iner.java:4168)
at java.awt.Container.dispatchEventImpl(Container.jav a:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2478 )
at java.awt.Component.dispatchEvent(Component.java:44 81)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.j ava:643)
at java.awt.EventQueue.access$000(EventQueue.java:84)
at java.awt.EventQueue$1.run(EventQueue.java:602)
at java.awt.EventQueue$1.run(EventQueue.java:600)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:87)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:98)
at java.awt.EventQueue$2.run(EventQueue.java:616)
at java.awt.EventQueue$2.run(EventQueue.java:614)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectio nPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java: 613)
at java.awt.EventDispatchThread.pumpOneEventForFilter s(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(E ventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarch y(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispa tchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThre ad.java:122)

import java.awt.event.*;
import javax.swing.*;
import java.awt.*;
import java.io.*;
public class primeGui implements ActionListener {
    JFrame frame; JPanel panel; JButton button; JLabel label, answer, highPrime; JTextField enterInfo;
    public primeGui() {
        frame = new JFrame("Prime Number Check");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        panel = new JPanel();
        panel.setLayout(new GridLayout(0,2,10,5));
        panel.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));
        label = new JLabel("Enter an integer:                       ");
        panel.add(label);
        enterInfo = new JTextField(10);
        panel.add(enterInfo);
        answer = new JLabel(" ");
        panel.add(answer);
        button = new JButton("check");
        button.setActionCommand("check");
        button.addActionListener(this);
        panel.add(button);
        highPrime = new JLabel(" ");
        panel.add(highPrime);
        frame.setContentPane(panel);
        frame.pack();
        frame.setVisible(true);
    }
    public void actionPerformed(ActionEvent event) {
        String eventName = event.getActionCommand();
        int counter=2; double number = 0; String prime = enterInfo.getText();
        String numInFile;
        if (eventName.equals("check")) {
             number = Double.parseDouble(enterInfo.getText());
            do {
                counter++;
            } while (number%counter!= 0);
            if (counter == Double.parseDouble(prime)) {
                answer.setText("The number IS prime.");
            }else{
                answer.setText("The number is NOT prime.");
            }
        }
        File prime1 = new File("prime.txt");
        if(!prime1.exists()) {
            try {
            prime1.createNewFile();
        }catch (IOException e) {
            highPrime.setText("Problem creating file.");
        }
        }
        try {
            FileReader in = new FileReader(prime1);
            BufferedReader readFile = new BufferedReader(in);
            FileWriter out = new FileWriter(prime1);
            BufferedWriter writeFile = new BufferedWriter(out);
            if ((numInFile = readFile.readLine()) == null) {
                writeFile.write(String.valueOf(number));
            }else{
                numInFile = readFile.readLine();
                if (number > Double.parseDouble(numInFile)) {
                    writeFile.write(String.valueOf(number));
                }
            }
            highPrime.setText("The highest prime ever tested is " + Double.parseDouble(readFile.readLine()));
            writeFile.close();out.close();readFile.close();in.close();
        }catch (FileNotFoundException e) {
            highPrime.setText("File not found.");
        }catch (IOException e) {
            highPrime.setText("Problem setting data.");
        }
    }
    private static void runGui() {
        primeGui gui = new primeGui();
    }
    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                runGui();
            }
        });
    }
}