Collision Detection returns ConcurrentModificationException

Question

I am currently working on a simple 2D graphics game. I just added collision detection and although it works, it returns ConcurrentModificationException . These are the errors I get.

My game also doesn't pause when the mouse exits the screen. Moreover, whenever a collision happens there is also a 1/2 to 1 second lag in the game which could be a result of the errors. Also, when I hit all of the bubbles, the bubbles do not continue to be drawn. Also, if you find any efficiency mistakes, please let me know. I have included the relevant code below. Thank you so much.

public class Game extends JPanel{

  public static final int WINDOW_WIDTH = 600;
  public static final int WINDOW_HEIGHT = 400;

  private boolean insideCircle = false;
  private static boolean ifPaused = false;
  private static JPanel mainPanel;
  private static JLabel statusbar;
  private static int clicks = 0;
  public static Bubbles[] BubblesArray = new Bubbles[5];
  public static List<Bubbles> bubbles = new ArrayList<Bubbles>();

public Game() {
  for (int i = 0; i < 10 ; i++){
    bubbles.add(new Bubbles());
  }
}

public void paint(Graphics graphics) {

  graphics.setColor(Color.WHITE);
  graphics.fillRect(0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);

  if (!ifPaused){
    for(Bubbles b : bubbles) b.paint(graphics);
  }
}

public void update() {

  if (!ifPaused){
    for (Bubbles b : bubbles) b.update();
  }
}

public static void main(String[] args) throws InterruptedException {
  statusbar = new JLabel("Default");   
  Game game = new Game();
  JFrame frame = new JFrame();
  game.addMouseMotionListener(new MouseAdapter() {
    @Override
    public void mouseMoved(MouseEvent e) {
      statusbar.setText(String.format("Your mouse is at %d, %d", e.getX(), e.getY()));
      for (Bubbles b : bubbles)
      {
        int x = b.getX();
        int y = b.getY();
        int r = b.getCircleSize();
        if ( (e.getX() >= (x - r/2) && e.getX() <= (x+ r/2)) && (e.getY() >= (y - r/2)  && e.getY() <= (y+ r/2)) ){
          //System.out.println("Inside");
          bubbles.remove(b);
        }
      }
    }

    public void mouseExited(MouseEvent e){
      ifPaused = true;
    }   

    public void mouseEntered(MouseEvent e){
      ifPaused = false;
    }
  });
  frame.add(game);
  frame.add(statusbar, BorderLayout.SOUTH);
  frame.setVisible(true);
  frame.setSize(WINDOW_WIDTH, WINDOW_HEIGHT);
  frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
  frame.setTitle("Bubble Burst");
  frame.setResizable(false);
  frame.setLocationRelativeTo(null);

  while (true) {
    if (!ifPaused){
      game.update();
      game.repaint();
      Thread.sleep(5);
    }
  }
}    

}