JFrame的repaint()无效

Question

package com.elyong.noway;

/**
 * Created by ely ong on 2017/11/27.
 */
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class SimpleGui3C implements ActionListener{
    JFrame frame;
    public static void main(String[] args){
        SimpleGui3C gui=new SimpleGui3C();
        gui.go();
    }
    public void go(){
        JFrame frame=new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JButton button=new JButton("Change colors");
        button.addActionListener(this);
        MyDrawPanel drawPanel=new MyDrawPanel();
        frame.getContentPane().add(BorderLayout.SOUTH,button);
        frame.getContentPane().add(BorderLayout.CENTER,drawPanel);
        frame.setSize(300,300);
        frame.setVisible(true);
    }
    public void actionPerformed(ActionEvent event){
        frame.repaint();
    }
}
class MyDrawPanel extends JPanel{
    public void paintComponent(Graphics g){
        Graphics2D g2d=(Graphics2D) g;
        int red=(int)(Math.random()*255);
        int green=(int)(Math.random()*255);
        int blue=(int)(Math.random()*255);
        Color startColor=new Color(red,green,blue);
        red=(int)(Math.random()*255);
        green=(int)(Math.random()*255);
        blue=(int)(Math.random()*255);
        Color endColor=new Color(red,green,blue);
        GradientPaint gradient=new GradientPaint(70,70,startColor,150,150,endColor);
        g2d.setPaint(gradient);
        g2d.fillOval(70,70,100,100);
    }
}

目的是写一个按下按钮圆圈就会改变颜色的程序。
错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at com.elyong.noway.SimpleGui3C.actionPerformed(SimpleGui3C.java:27)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2348)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
    at java.awt.Component.processMouseEvent(Component.java:6533)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3324)
    at java.awt.Component.processEvent(Component.java:6298)
    at java.awt.Container.processEvent(Container.java:2236)
    at java.awt.Component.dispatchEventImpl(Component.java:4889)
    at java.awt.Container.dispatchEventImpl(Container.java:2294)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466)
    at java.awt.Container.dispatchEventImpl(Container.java:2280)
    at java.awt.Window.dispatchEventImpl(Window.java:2746)
    at java.awt.Component.dispatchEvent(Component.java:4711)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758)
    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.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:90)
    at java.awt.EventQueue$4.run(EventQueue.java:731)
    at java.awt.EventQueue$4.run(EventQueue.java:729)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:80)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:728)
    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)