Java SE (Java Platform, Standard Edition)

This is my code:

import javax.swing.JFrame;

public class MainFrame extends JFrame {
	public MainFrame(){
		init();
	}
	
	public void init(){
		setSize(100,100);
	}
}


import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;

public class SubFrame extends MainFrame implements ActionListener{
	private JButton button1 = null;
	public SubFrame(){
		super();
		
	}
	public void init(){
		super.init();
		button1 = new JButton("button1");
		add(button1);
		button1.addActionListener(this);
	}
	public void actionPerformed(ActionEvent e) {
		// TODO Auto-generated method stub
		button1.setEnabled(false);
	}
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		(new SubFrame()).setVisible(true);
	}

}

When I click the button, this exception will be thrown:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at swing.SubFrame.actionPerformed(SubFrame.java:23)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
at java.awt.Component.processMouseEvent(Component.java:6038)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3265)
at java.awt.Component.processEvent(Component.java:5803)
at java.awt.Container.processEvent(Container.java:2058)
at java.awt.Component.dispatchEventImpl(Component.java:4410)
at java.awt.Container.dispatchEventImpl(Container.java:2116)
at java.awt.Component.dispatchEvent(Component.java:4240)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
at java.awt.Container.dispatchEventImpl(Container.java:2102)
at java.awt.Window.dispatchEventImpl(Window.java:2429)
at java.awt.Component.dispatchEvent(Component.java:4240)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)