无法使用super.paintComponent(g)在JFrame中显示JPanel。Java中的空JFrame

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

class MyPanel extends JPanel {
    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        // 在这里进行自定义的绘制操作
    }
}

class MainWindow extends JFrame {
    public MainWindow() {
        setSize(400, 300);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new BorderLayout());

        MyPanel panel = new MyPanel();
        add(panel, BorderLayout.CENTER);
    }
}

public class Main {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            MainWindow window = new MainWindow();
            window.setVisible(true);
        });
    }
}