loading new content on same window

I am trying to load another pane but within the same window. i have 2 basic buy/sell buttons and when user clicks on either button it clears the current one but loads new buttons, labels, etc.

I already set up the forms for the buy and sell screens i want i only linked the main first screen with a class.

This is the code i have so far.

[code]

package guistuff;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class GuiTest {
    private JButton btnpurchase;
    private JPanel panelMain;
    private JButton btnrefund;

    private JPanel panelPurchase;
    private JLabel lblnullbuy;
    private JLabel lblamtbuy;

    private JPanel panelRefund;
    private JLabel lblnullsell;
    private JLabel lblamtsell;

    public GuiTest() {
        btnpurchase.addActionListener(new ActionListener(){
            @Override
            public void actionPerformed(ActionEvent e) {
                Component component = (Component) e.getSource();
                JFrame frame = (JFrame) SwingUtilities.getRoot(component);
                frame.setContentPane(new GuiTest().panelPurchase);
                frame.setVisible(true);
            }
        });
        btnrefund.addActionListener(new ActionListener(){
            @Override
            public void actionPerformed(ActionEvent e) {
                Component component = (Component) e.getSource();
                JFrame frame = (JFrame) SwingUtilities.getRoot(component);
                frame.setContentPane(new GuiTest().panelRefund);
            }
        });
    }
    public void purchasecontent(){

    }
    public void refundcontent(){

    }
    public static void main(String[] args){
        JFrame frame = new JFrame("GuiTest");
        frame.setContentPane(new GuiTest().panelMain);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.pack();
        frame.setSize(480,320);
        frame.setVisible(true);
    }
}
[/code]

when i click on the button refund it spits out an error...

Exception in thread "AWT-EventQueue-0" java.awt.IllegalComponentStateException: contentPane cannot be set to null.
 at javax.swing.JRootPane.setContentPane(JRootPane.java:621)
 at javax.swing.JFrame.setContentPane(JFrame.java:698)
 at guistuff.GuiTest$2.actionPerformed(GuiTest.java:35)
 at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2022)
 at javax.swing.AbstractButton$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)