Выдает ошибку не могу понять почему

package AllClass;
 
import javax.swing.JFrame;
 
public class StartGame {
 
    public static void main(String[] args) {
 
        
    Proccess proccess = new Proccess();
    proccess.initBall();
        
    }
 
}

package AllClass;
 
import java.awt.Color;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
 
import javax.swing.BorderFactory;
import javax.swing.Icon;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
 
public class GameWindow extends JFrame implements MouseListener{
    
    Proccess proccess;
 
    private JLabel[][] game_field = new JLabel[9][9];
    private JLabel templabel = new JLabel();
    private JPanel game_field_panel = new JPanel();
    private Icon balls[] = { new ImageIcon("blueball.png"), new ImageIcon("greenball.png"),
            new ImageIcon("redball.png") };
 
    private boolean set_end_game = false;
    private boolean delete_ball = false;
    
    public GameWindow() {
        
        setLayout(new FlowLayout());
        game_field_panel.setLayout(new GridLayout(9, 9));
        
        for (int i = 0; i < game_field.length; i++) {
            for (int j = 0; j < game_field[i].length; j++) {
                game_field[i][j] = new JLabel();
                game_field[i][j].setVerticalAlignment(JLabel.CENTER);
                game_field[i][j].setHorizontalAlignment(JLabel.CENTER);
                game_field[i][j].setBorder(BorderFactory.createLineBorder(Color.BLACK, 1));
                game_field_panel.add(game_field[i][j]);
                game_field[i][j].addMouseListener(this);
            }
        }
 
        game_field_panel.setPreferredSize(new Dimension(400, 400));
        add(game_field_panel);
 
        setLocation(330, 150);
        setSize(500, 500);
        setVisible(true);
    }
    
    public GameWindow(Proccess proccess) {
        this.proccess = proccess;
    }
    
    public JLabel[][] getGame_field() {
        return game_field;
    }
 
    public void setGame_field(JLabel[][] game_field) {
        this.game_field = game_field;
    }
 
    public JLabel getTemplabel() {
        return templabel;
    }
 
    public void setTemplabel(JLabel templabel) {
        this.templabel = templabel;
    }
 
    public JPanel getGame_field_panel() {
        return game_field_panel;
    }
 
    public void setGame_field_panel(JPanel game_field_panel) {
        this.game_field_panel = game_field_panel;
    }
 
    public Icon[] getBalls() {
        return balls;
    }
 
    public void setBalls(Icon[] balls) {
        this.balls = balls;
    }
 
    public boolean isSet_end_game() {
        return set_end_game;
    }
 
    public void setSet_end_game(boolean set_end_game) {
        this.set_end_game = set_end_game;
    }
 
    public boolean isDelete_ball() {
        return delete_ball;
    }
 
    public void setDelete_ball(boolean delete_ball) {
        this.delete_ball = delete_ball;
    }
 
    @Override
    public void mouseClicked(MouseEvent e) {
        for (int i = 0; i < game_field.length; i++) {
            for (int j = 0; j < game_field[i].length; j++) {
                if (e.getSource() == game_field[i][j] && delete_ball == false) {
                    if (game_field[i][j].getIcon() != null && templabel.getIcon() == null) {
                        templabel.setIcon(game_field[i][j].getIcon());
                        game_field[i][j].setIcon(null);
                    } else if (game_field[i][j].getIcon() == null && templabel.getIcon() != null) {
                        game_field[i][j].setIcon(templabel.getIcon());
                        templabel.setIcon(null);
                        if (set_end_game == false) {
                            for (int k = 0; k < 3; k++) {
                                proccess.setNewBall();
                            }
                        }
                    } else {
                    }
                } else {
                    delete_ball = false;
                }
            }
        }
    }
 
    @Override
    public void mousePressed(MouseEvent e) {
    }
 
    @Override
    public void mouseReleased(MouseEvent e) {
    }
 
    @Override
    public void mouseEntered(MouseEvent e) {
    }
 
    @Override
    public void mouseExited(MouseEvent e) {
    }
}

package AllClass;
 
public class Proccess {
 
    GameWindow gameWindow = new GameWindow();
    
    public Proccess() {
        
    }
    
    public void initBall () {
        for (int i = 0; i < 3; i++) {
            int cell_ver_cord = (int) (Math.random() * 9);
            int cell_hor_cord = (int) (Math.random() * 9);
            int random_ball = (int) (Math.random() * 3);
            gameWindow.getGame_field()[cell_hor_cord][cell_ver_cord].setIcon(gameWindow.getBalls()[random_ball]);
        }
    }
    
    public void setNewBall() {
        int cell_ver_cord = (int) (Math.random() * 9);
        int cell_hor_cord = (int) (Math.random() * 9);
        int random_ball = (int) (Math.random() * 3);
        if (gameWindow.getGame_field()[cell_hor_cord][cell_ver_cord].getIcon() == null) {
            gameWindow.getGame_field()[cell_hor_cord][cell_ver_cord].setIcon(gameWindow.getBalls()[random_ball]);
        } else {
            setNewBall();
        }
    }
    
}