Игрушка "Поймай каплю". Не работает кнопка restart

package lesson_6_7_8.project_1.catch_the_drop;
 
import javax.imageio.ImageIO;
import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.io.IOException;
 
public class GameWindow extends JFrame {
 
    private static GameWindow game_window;
    private static long last_frame_time;
    private static Image background_game;
    private static Image game_over;
    private static Image drop;
    private static Image game;
    private static boolean game_o = false;
    private static float game_left = 0;
    private static float game_top = 0;
    private static float game_right = 247;
    private static float game_bottom = 50;
    private static float drop_left = 200;
    private static float drop_top = -100;
    private static float drop_v = 200;
    private static int score;
 
    public static void main(String[] args) throws IOException {
        background_game = ImageIO.read(GameWindow.class.getResourceAsStream("background_game.png"));
        game_over = ImageIO.read(GameWindow.class.getResourceAsStream("game_over.png"));
        drop = ImageIO.read(GameWindow.class.getResourceAsStream("drop.png"));
        game = ImageIO.read(GameWindow.class.getResourceAsStream("game.jpg"));
        game_window = new GameWindow();
        game_window.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        game_window.setLocation(200, 100);
        game_window.setSize(906, 478);
        game_window.setResizable(false);
        last_frame_time = System.nanoTime();
        GameField game_field = new GameField ();
        game_field.addMouseListener (new MouseAdapter()  {
            @Override
            public void mousePressed(MouseEvent e) {
                int x = e.getX();
                int y = e.getY();
                float drop_right = drop_left + drop.getWidth(null);
                float drop_bottom = drop_top + drop.getHeight(null);
                boolean is_drop = x >= drop_left && x <= drop_right && y >= drop_top && y <= drop_bottom;
                if (is_drop) {
                    drop_top = -100;
                    drop_left = (int) (Math.random() * (game_field.getWidth() - drop.getWidth(null)));
                    drop_v = drop_v + 5;
                    score++;
                    game_window.setTitle ("Score: " + score);
                }
                boolean is_game = x >= game_left && x <= game_right && y >= game_top && y <= game_bottom;
                if (is_game) {
                    game_o = false;
                    drop_v = 200;
                    score = 0;
                }
 
            }
        });
        game_window.add(game_field);
        game_window.setVisible(true);
    }
 
    private static void onRepaint (Graphics g) {
        long current_time = System.nanoTime();
        float delta_time = (current_time - last_frame_time) * 0.000000001f;
        last_frame_time = current_time;
 
        drop_top = drop_top + drop_v * delta_time;
        //drop_left = drop_left + drop_v * delta_time;
        g.drawImage(background_game, 0, 0, null );
        g.drawImage(drop, (int) drop_left, (int) drop_top, null);
        if (drop_top > game_window.getHeight()) {g.drawImage(game_over, 280, 120, null);
            game_o = true;
        g.drawImage (game, 0, 0, null);}
 
    }
 
    private static class GameField extends JPanel {
        //динамическое замещение
        @Override
        protected void paintComponent (Graphics g){
            super.paintComponent (g);
                onRepaint (g);
                repaint ();
 
        }
    }
}