NullPointerException on addPropertyChangeListener

NullPointerException on addPropertyChangeListener

I have created a simple LED that receives an input from any of a number of
digital components such as switches/gates. The problem is that when trying
to implement the PropertyChangeListener interface I get a
NullPointerException. With the code below if I just add this to a JFrame
form and try and run it I get this exception. I have implemented the LED
the same as I did for gates/switches however for some reason my code
generates an error. Any help appreciated.
package Digital;
import java.awt.Image;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.io.Serializable;
public class LED extends javax.swing.JPanel implements
PropertyChangeListener {
private Image led_on;
private Image led_off;
private Image image;
private Terminal input;
private transient PropertyChangeSupport propertyChangeSupport = new
PropertyChangeSupport(this);
public LED() {
java.net.URL url_on = getClass().getResource("images/LED_on.gif");
led_on = new javax.swing.ImageIcon(url_on).getImage();
this.setSize(led_on.getWidth(null), led_on.getHeight(null));
java.net.URL url_off = getClass().getResource("images/LED_off.gif");
led_off = new javax.swing.ImageIcon(url_off).getImage();
this.setSize(led_off.getWidth(null), led_off.getHeight(null));
this.image = led_off;
}
@Override
public void paintComponent(java.awt.Graphics g) {
g.drawImage(image, 0, 0, null);
}
public static final String PROP_INPUT = "input";
public Terminal getInput() {
return input;
}
public void setInput(Terminal input) {
if (input != null) {
input.addPropertyChangeListener(this);
this.addPropertyChangeListener(this);
}
Terminal oldInput = this.input;
this.input = input;
propertyChangeSupport.firePropertyChange(PROP_INPUT, oldInput,
input);
}
public void addPropertyChangeListener(PropertyChangeListener listener) {
propertyChangeSupport.addPropertyChangeListener(listener);
}
public void removePropertyChangeListener(PropertyChangeListener
listener) {
propertyChangeSupport.removePropertyChangeListener(listener);
}
public boolean Recalculate(Terminal input) {
if (input!=null) {
return input.getValue();
} else {
return false;
}
}
public void ledChange(boolean ledValue) {
if (ledValue) {
image = led_on;
} else {
image = led_off;
}
repaint();
}
public void propertyChange(PropertyChangeEvent pce) {
boolean terminalValue = Recalculate(input);
ledChange(terminalValue);
}
}