Add default close operation to terminate demo

Author: mikera

src/main/java/MaterialUISwingDemo.java

@@ -328,7 +328,8 @@ public void actionPerformed(ActionEvent e) {
 // make everything visible to the world
 frame.pack ();
 frame.setVisible (true);
-
+frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
+
 JFileChooser fileChooser = new JFileChooser ();
 fileChooser.showOpenDialog (frame);
 }