Fixed NPE reported through a bug report. Somehow the listenerList was

null when a change event was fired.  Don't know how this really can
happen, but protecting against the NPE isn't a bad idea.

core/src/net/sf/openrocket/rocketcomponent/Configuration.java

@@ -189,11 +189,10 @@ public class Configuration implements Cloneable, ChangeSource, ComponentChangeLi
 	 */
 	public void release() {
 		rocket.removeComponentChangeListener(this);
-		listenerList = null;
+		listenerList = new ArrayList<EventListener>();
 		rocket = null;
 	}
 	
-	
 	////////////////  Listeners  ////////////////
 	
 	@Override