基于继承的小型Java GUI程序无法正常显示问题求助

我是Java新手（这是我第一次在校外编写程序），目前遇到GUI无法正常打开的问题。奇怪的是，程序偶尔能正常显示并运行（大概10次里有1次），但大多数情况下JFrame打开后内部没有任何可见内容。

相关代码类 #

GUI.java #

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridLayout;
import javax.swing.*;
import javax.swing.border.Border;

public class GUI {
    
    JFrame frame;
    JPanel panel;
    
    public GUI() {
        frame = new JFrame();
        frame.setTitle("Word Game");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(500, 500);
        frame.setResizable(false);
        frame.setVisible(true);
        
        panel = new JPanel();
        panel.setBackground(Color.red);
        panel.setBorder(BorderFactory.createEmptyBorder(30, 30, 10, 30));
        panel.setLayout(new GridLayout(1, 0));
    }
}

MainMenu.java #

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.border.Border;

public class MainMenu extends GUI implements ActionListener {

    private JButton buttonPlay;
    private JButton buttonStats;
    
    public MainMenu() {
        super();
        
        buttonPlay = new JButton("Play");
        panel.add(buttonPlay);
        buttonPlay.addActionListener(this);
        buttonStats = new JButton("View Leaderboards");
        panel.add(buttonStats);
        buttonStats.addActionListener(this);
        
        frame.add(panel, BorderLayout.CENTER);
    }
    
    @Override
    public void actionPerformed(ActionEvent e) {
        new SelectLevel(e.getSource() == buttonPlay);
        frame.dispose();
    }

    public static void main(String[] args) {
        new MainMenu();
    }
}

SelectLevel.java #

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.*;
import javax.swing.border.Border;

public class SelectLevel extends GUI implements ActionListener {

    
    public SelectLevel(boolean play) {
        super();
        
        for(int i = 1; i <= 3; i++) {
            JButton button = new JButton("Level " + i);
            panel.add(button);
            button.addActionListener(this); 
        }
        
        frame.add(panel, BorderLayout.CENTER);
    }
    
    @Override
    public void actionPerformed(ActionEvent e) {
        new Game(e.getActionCommand().charAt(6));
        frame.dispose();
    }

    public static void main(String[] args) {
        new MainMenu();
    }
}

界面截图 #

正常显示界面 #

• 主菜单界面：
• 选关界面：

异常显示界面 #

内容的提问来源于stack exchange，提问作者Piter De Vries