swing怎么在一个程序中让不同的组件显示不同的外观风格呀

swing怎么在一个程序中让不同的组件显示不同的外观风格呀
我自己写了一个小程序，但是运行时，当我通过点击程序中更改外观的按钮后，抛出了一大堆异常
哪位大牛能帮我看下是哪里出了问题
我总共写了两个类，一个是MainFrame：继承JFrame，主要就是创建主程序并将其显示出来
另一个是PreviewPanel：继承JPanel，用来容纳需要更改外观的组件
下面是代码：
MainFrame：public class MainFrame extends JFrame {    private JPanel contentPane;
    private PreviewPanel previewPanel;    /**
     * main方法
     */
    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                //在frame初始化之前先设置外观
                try {
                    UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
                    MainFrame frame = new MainFrame();
                    frame.setVisible(true);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }    /**
     * Create the frame.
     */
    public MainFrame() {
        previewPanel = new PreviewPanel();
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 452, 436);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(new BorderLayout(0, 0));  //设置contentPane为BorderLayout        JButton btn1 = new JButton("预览");
        contentPane.add(btn1, BorderLayout.NORTH);      //添加名为“预览”的按钮
        btn1.addActionListener(new ActionListener() {   //-
            public void actionPerformed(ActionEvent e) {
                previewPanel.setLookAndFeel();          //
            }
        });        contentPane.add(previewPanel);                  //添加预览面板
    }
}
PreviewPanel：public class PreviewPanel extends JPanel {
    JButton previewBtn; // 被预览的按钮    /**
     * 构造函数： 向面板添加JTabbedPane，并在JTabbedPane中添加放置了previewBtn按钮的buttonPanel面板
     */
    public PreviewPanel() {
        setLayout(new BoxLayout(this, BoxLayout.X_AXIS));        JTabbedPane tabbedPane = new JTabbedPane(JTabbedPane.TOP);
        add(tabbedPane);        JPanel buttonPanel = new JPanel();
        tabbedPane.addTab("JButton", null, buttonPanel, null);        previewBtn = new JButton("JButton");
        buttonPanel.add(previewBtn);
    }    /**
     * 设置外观的方法
     */
    public void setLookAndFeel() {
        try {
            SynthLookAndFeel synth = new SynthLookAndFeel();
            // 加载配置文件demo.xml
            synth.load(new URL("file:///E:\\EEWork\\Apem\\eclipseWork1"
            + "\\synth\\src\\demo\\synth\\demo.xml"));
            UIManager.setLookAndFeel(synth);
        } catch (ParseException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (UnsupportedLookAndFeelException e) {
            e.printStackTrace();
        }        SwingUtilities.updateComponentTreeUI(previewBtn);// 更新previewBtn按钮的外观
    }
}
下面是程序设置外观过程中报的异常：Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at com.sun.java.swing.plaf.windows.WindowsTabbedPaneUI.paintContentBorder(WindowsTabbedPaneUI.java:145)
at javax.swing.plaf.basic.BasicTabbedPaneUI.paint(BasicTabbedPaneUI.java:791)
at javax.swing.plaf.ComponentUI.update(ComponentUI.java:161)
at javax.swing.JComponent.paintComponent(JComponent.java:780)
at javax.swing.JComponent.paint(JComponent.java:1056)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)
at javax.swing.JLayeredPane.paint(JLayeredPane.java:586)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paintToOffscreen(JComponent.java:5217)
at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(RepaintManager.java:1579)
at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1502)
at javax.swing.RepaintManager.paint(RepaintManager.java:1272)
at javax.swing.JComponent.paint(JComponent.java:1042)
at java.awt.GraphicsCallback$PaintCallback.run(GraphicsCallback.java:39)
at sun.awt.SunGraphicsCallback.runOneComponent(SunGraphicsCallback.java:79)
at sun.awt.SunGraphicsCallback.runComponents(SunGraphicsCallback.java:116)
at java.awt.Container.paint(Container.java:1976)
at java.awt.Window.paint(Window.java:3906)
at javax.swing.RepaintManager$4.run(RepaintManager.java:842)
at javax.swing.RepaintManager$4.run(RepaintManager.java:814)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:814)
at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:789)
at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:738)
at javax.swing.RepaintManager.access$1200(RepaintManager.java:64)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1732)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:756)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:709)
at java.awt.EventQueue$3.run(EventQueue.java:703)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:726)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)