The code that should work

CustomizeWindow.java

import java.awt.*;
import javax.swing.*;

public class CustomizeWindow extends JFrame{
  /**
   * 
	 */
	private static final long serialVersionUID = 1L;

	public static void main(String[]args){
		JFrame mainFrame = new JFrame();
		Customize c = new Customize();
    		c.setGUI();                                 // <-------------- The code forgot to call this line.
		JPanel panel  = new JPanel();
		panel.add(c);
		mainFrame.getContentPane().add(panel);
//		mainFrame.setPreferredSize(new Dimension(1000,1000));
		mainFrame.setSize(new Dimension(1000,1000));
		mainFrame.setVisible(true);
		mainFrame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);

	}

}