IllegalArgumentException: adding window to contentPane

Question

That's the exception I get on running this, but itdoesn't kill the program. Also, the switchColor() function changes the Box color to red the first time, but does not toggle. I've spent way longer on this then I'm proud to say.
Please help.
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class WindowButton1
public static void main(String[] args)
     window = new MainWindow("Action example");
     Toolkit theKit = window.getToolkit();          
     Dimension wndSize = theKit.getScreenSize();  
     window.setBounds(0, 0,wndSize.width/2, wndSize.height/2);
     window.addWindowListener(new WindowAdapter()     
          // Handler for window closing event
          public void windowClosing(WindowEvent e)
               window.dispose();                          
               System.exit(0);                            
          window.setVisible(true); // Display the window
     static MainWindow window;          
}//end WindowButton1
class MainWindow extends JFrame
     MainWindow(String title)
     super(title);
     setJMenuBar(menuBar);                  
     fileMenu = new JMenu("File");
     connectItem = new JMenuItem();                                        
     connectItem = fileMenu.add("Connect...");
     connectItem.addActionListener(menuHandler = new Handler());
     menuBar.add(fileMenu);           
}//end constructor
//innerclass to handle "Connect" menu action
class Handler implements ActionListener
     public void actionPerformed(ActionEvent e)
      win = new ConnectWindow("ColorButton Test");
      getContentPane().add(win,BorderLayout.CENTER);
      setVisible(true);
}//end inner class
private JMenu fileMenu;
private JMenuItem connectItem;
private JMenuBar menuBar = new JMenuBar(); 
private Handler menuHandler;
private ConnectWindow win;
//separate class to display buttonBox
class ConnectWindow extends JFrame
ConnectWindow(String title)
     setTitle("Action Buttons");
     Toolkit kit = Toolkit.getDefaultToolkit();
     Dimension screenSize = kit.getScreenSize();
     int screenHeight = screenSize.height;
     int screenWidth = screenSize.width;
     setSize(screenWidth / 3, screenHeight / 3);
     setLocation(screenWidth / 4, screenHeight / 4);
//buttons to go into panel
     JButton button1 = new JButton("Color Change");
     JButton button2 = new JButton("Cancel");
     JButton button3 = new JButton("Advanced");
     Dimension buttonSize = new Dimension(100,20);
     button1.setPreferredSize(buttonSize);
     button2.setPreferredSize(buttonSize);
     button3.setPreferredSize(buttonSize);
//Event handling for the three buttons
     button1.addActionListener(new ActionListener()
     public void actionPerformed(ActionEvent e)
     switchColor();
     repaint();
//Cancel button closes window
     button2.addActionListener(new ActionListener()
     public void actionPerformed(ActionEvent e)
     dispose();                            
//Advanced button is dummy for now
     button3.addActionListener(new ActionListener()
     public void actionPerformed(ActionEvent e)
//place buttons in panel
     Box buttonBox = Box.createVerticalBox();
     buttonBox.add(button1);
     buttonBox.add(button2);
     buttonBox.add(button3);
     //place button box in main window
     getContentPane().add(buttonBox, BorderLayout.CENTER);
     setVisible(true);
     }//end constructor
     public void switchColor()
          isRed = false;
          if (!(isRed))
          getContentPane().setBackground(Color.red);
          else getContentPane().setBackground(Color.white);
          setVisible(true);
          repaint();
          isRed = !(isRed);
     boolean isRed;
     private Box buttonBox;
     private JButton button1, button2, button3;
}//end ConnectWindow

Answer 1

Change ConnectWindow to
class ConnectWindow extends JInternalFrameHope this helps youThanks! The last piece is toggling the colors. Here's my actionPerformed method, and the method it calls (taken from the above code:
button1.addActionListener(new ActionListener()
public void actionPerformed(ActionEvent e)
switchColor();
repaint();
public void switchColor()
isRed = false;
if (!(isRed))
WindowButton1.window.getContentPane().setBackground(Color.red);
else getContentPane().setBackground(Color.white);
setVisible(true);
repaint();
isRed = !(isRed);
boolean isRed;