// Textbook fragment 02.08
java.lang.NullPointerException: Returned a null locator
   at java.awt.Component.handleEvent(Component.java:900)
   at java.awt.Component.postEvent(Component.java:838)
   at java.awt.Component.postEvent(Component.java:845)
   at sun.awt.motif.MButtonPeer.action(MButtonPeer.java:39)
   at java.lang.Thread.run(Thread.java)