Panels not visible until user clicks

Question

I just recently started learning Java.

Playing around with graphics I wrote the test code below which at program start should display a frame with a central panel for graphics and an upper panel with a menu. However, an empty frame is shown instead. Then, as soon as the user mouse clicks on the frame everything shows up as intended.

What am I doing wrong?

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.EventQueue;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.JFrame;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JPanel;

public class Demo {

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() { 
                theFrame f = new theFrame("Learning practice",50,50,600,400);       
                f.setResizable(false);                  
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                f.setLocationRelativeTo(null);
                f.setVisible(true);             
            }
        });
    }
}

class theFrame extends JFrame { 
    private boolean gridIsOn;
    private Point point;
    private int myWidth,myHeight;
    public theFrame(String title,int left,int top,int width,int height) {
        gridIsOn=false;
        myWidth=width;
        myHeight=height;    
        setTitle(title);
        setLocation(left,top);
        setSize(width,height);
        upperPanel up=new upperPanel();
        add (up, BorderLayout.NORTH);
        centralPanel cp=new centralPanel();
        cp.setBackground(Color.LIGHT_GRAY);
        add(cp,BorderLayout.CENTER);    
    }
    
    class upperPanel extends JPanel {
        public upperPanel() {
            setLayout(new FlowLayout(FlowLayout.LEFT));
            JMenuBar myMenuBar=new JMenuBar();
            JMenu tools=new JMenu("Tools");
            JMenuItem grid=new JMenuItem("Toggle grid");
            grid.addActionListener(new MenuActList());
            myMenuBar.add(tools);
            tools.add(grid);
            add(myMenuBar); 
        }
    }
    
    class MenuActList implements ActionListener {
        public void actionPerformed(ActionEvent e) {        
            if (e.getActionCommand()=="Toggle grid") {
                gridIsOn=!gridIsOn;
                repaint();
            }
        }
    }
    
    class centralPanel extends JPanel {
        public centralPanel() {
            staticMouse Jerry=new staticMouse();
            addMouseListener(Jerry);
        }
        
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2 = (Graphics2D) g;                 
            
            if(gridIsOn) {
                g2.setColor(Color.BLUE);
                for(int i=0;i<myWidth;i+=10) {
                    g2.drawLine(i, 0, i, myHeight);
                }
                for(int i=0;i<myHeight;i+=10) {
                    g2.drawLine(0, i, myWidth, i);
                }
            }
            
            g2.setColor(Color.RED);
            g2.drawRect(point.x-25,point.y-25,50,50);

        }           

    }

    class staticMouse implements MouseListener {

        @Override
        public void mouseClicked(MouseEvent e) {
            point = e.getPoint();
            repaint();
        }

        @Override
        public void mouseEntered(MouseEvent arg0) {}
        @Override
        public void mouseExited(MouseEvent arg0) {}
        @Override
        public void mousePressed(MouseEvent e) {}
        @Override
        public void mouseReleased(MouseEvent arg0) {}
        
    }
    
}