Suppression d'un objet d'une collection lors de son itération

J'essaye de supprimer les objets qui sont en dehors de JPanel. Cependant, lorsque je fais cela, j'obtiens cette erreur

et mon programme plante. Mon conférencier m'a dit que c'était parce que deux Threads accédaient à la ArrayList qui stockait mes objets.

J'ai fait pour synchroniser les fonctions mais ce n'est pas le cas ne fonctionne pas.

Bullet

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

public class Bullet {
    //Environment
    public static ArrayList<Bullet> bullet = new ArrayList<>();
    private String path = Application.path;
    private GamePanel gp;
    //properties
    private int x,y;
    private int width,height;
    private int yVector;
    private Image image;


    Bullet(GamePanel gp, int x, int y){
        image = new ImageIcon(path+"\\images\\javaicon.png").getImage();
        width=image.getWidth(null);
        height=image.getHeight(null);
        this.gp=gp;
        this.x=x;
        this.y=y;
        yVector=5;
    }

    public void move(){
        if(y< -height){
           bullet.remove(this);
        }
        y-=5;
    }

    public Image getImg(){
        return image;
    }

    public int getX(){
        return x;
    }

    public int getY(){
        return y;
    }
}

Classes pertinentes:

Application

import javax.swing.*;
import java.awt.*;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.security.Key;
import java.util.ArrayList;

public class GamePanel extends JPanel implements Runnable, KeyListener{
    private String path = Application.path;
    Image gameOverImg = new ImageIcon(path+"//images//gameover1.png").getImage();
    private Ihsan ihsan;
    private ArrayList <David> david = new ArrayList<>();
    private int enemies=5;
    private boolean pause=false;
    private boolean gameOver=false;

    GamePanel(){
        ihsan = new Ihsan(this);
        for(int i=0; i<enemies; i++){
            david.add(new David(this));
        }
        setFocusable(true);
        requestFocusInWindow();
        addKeyListener(this);
    }

    @Override
    public void run() {
        while (!pause){
            repaint();
            for(David david:david){
                david.move();
            }
            for(Bullet bullet:Bullet.bullet){
                bullet.move();
            }
            try{Thread.sleep(30);}
            catch (InterruptedException e){}
        }
    }

    public void paint(Graphics g){
        Graphics2D g2d = (Graphics2D) g.create();
        g2d.setColor(Color.GRAY);
        g2d.fillRect(0,0 ,getWidth(), getHeight());

        for(David david : david){
            g2d.drawImage(david.getImg(), david.getX(), david.getY(), null);
        }
        g2d.drawImage(ihsan.getImg(), ihsan.getX(), ihsan.getY(), null);
        for (Bullet bullet:Bullet.bullet){
            g2d.drawImage(bullet.getImg(), bullet.getX(), bullet.getY(), null);
        }

        if(gameOver){
            g2d.drawImage(gameOverImg,0,getHeight()/4,null);
        }
    }

    private static final Dimension DESIRED_SIZE = new Dimension(600,700);
    @Override
    public Dimension getPreferredSize(){
        return DESIRED_SIZE;
    }

    public void setGameOver(boolean gameOver) {
        this.gameOver = gameOver;
    }

    @Override
    public void keyPressed(KeyEvent e) {
        int key=e.getKeyCode();

        if (key==KeyEvent.VK_D || key==KeyEvent.VK_RIGHT){
            ihsan.move(4,0);
            System.out.println("Right Key");
        }

        if (key==KeyEvent.VK_A || key== KeyEvent.VK_LEFT){
            ihsan.move(-4,0);
            System.out.println("Left Key");
        }

        if(key==KeyEvent.VK_SPACE){
            Bullet.bullet.add(new Bullet(this,ihsan.getX()+(ihsan.getWidth()/2), ihsan.getY()));
        }
    }

    @Override
    public void keyTyped(KeyEvent e) { }

    @Override
    public void keyReleased(KeyEvent e) { }

    public boolean getGameOver(){
        return gameOver;
    }
}

GamePanel

import javax.swing.*;

public class Application {
    public static String path ="C:\\Users\\jarek\\OneDrive\\NUIG Private\\(2) Semester 2 2019\\Next Generation Technologies II CT255\\Assignment 3\\";
    private Application(){
        JFrame frame = new JFrame("Ihsan The Defender");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        GamePanel gamePanel= new GamePanel();
        frame.add(gamePanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setResizable(false);
        frame.setVisible(true);
        new Thread(gamePanel).start();
    }

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

Bullet strong >

public void move(){
        if(y< -height){
            synchronized (this) {
                bullet.remove(this);
            }
        }
        y-=5;
    }