renturn 错误

package ui.cfg;

import java.awt.BorderLayout;
import java.awt.FlowLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JTabbedPane;

import util.FrameUtil;

public class FrameConfig extends JFrame{
    
    private JButton btnOk = new JButton("确定");
    
    private JButton btnContrl = new JButton("取消");
    
    private JButton btnUser = new JButton("应用");
    
    public FrameConfig() {
        //设置布局管理器为“边界布局”
        this.setLayout(new BorderLayout());
        //添加主面板
        this.add(createMainPanel(),BorderLayout.CENTER);
        //添加按钮面板
        this.add(this.createButtonPanel(),BorderLayout.SOUTH);
        this.setSize(512, 256);
        //居中
        FrameUtil.setFrameCenter(this);
        //TODO测试用
        this.setDefaultCloseOperation(3);
        this.setVisible(true);
    }

    /**
     * 创建按钮面板
     */
    private JPanel createButtonPanel() {
        //创建按钮面板，流式布局
        JPanel jp = new JPanel(new FlowLayout(FlowLayout.RIGHT));
        jp.add(this.btnOk);
        jp.add(this.btnContrl);
        jp.add(this.btnUser);
        return jp;
    }

    /**
     * 创建主面板
     * @return
     */
    private JTabbedPane createMainPanel() {
        return null;
    }
    
    public static void main(String[] args) {
        new FrameConfig();
    }
    }

源码如上，下面是报错：

Exception in thread "main" java.lang.NullPointerException
	at java.awt.Container.addImpl(Unknown Source)
	at java.awt.Container.add(Unknown Source)
	at javax.swing.JFrame.addImpl(Unknown Source)
	at java.awt.Container.add(Unknown Source)
	at ui.cfg.FrameConfig.<init>(FrameConfig.java:25)
	at ui.cfg.FrameConfig.main(FrameConfig.java:57)
找了十多分钟，终于发现
 private JTabbedPane createMainPanel()
{ return null; } 
这个方法的renturn出错了，不应该return null，而应该return new JTabbedPane()才对，改完之后运行就没问题了。