LibGDX Crashing when removing entity from its list

Question

    public void collideAlien(Player player) {
        // get alien and bullet lists, loop through aliens and check if bullet collide
        // if collide, remove bullet and alien, play alien_hit sound
        List<Enemy> EnemyList = entityManager.getEnemyList();
        List<PlayerBullet> PlayerBulletList = entityManager.getPlayerbulletList();
        
        
        for (Iterator<Enemy> enemyIterator = EnemyList.iterator(); enemyIterator.hasNext();) {
            Enemy alien = enemyIterator.next();
            for (int j = 0; j < PlayerBulletList.size(); j++) {
                PlayerBullet bullet = PlayerBulletList.get(j);
                
                if (alien.isCollide(bullet)) {
                    alienHitSound.setVolume(audioSettings.getSoundVolume());
                    if (audioSettings.isSoundEnabled()) {
                        alienHitSound.play();
                    }
                    
                    if (alien.getEnemyType() == "empty") {
                        player.setScore(player.getScore() + 10);
                    }
                    
                    if (alien.getEnemyType() == "space") {
                        player.setScore(player.getScore() + 20);
                    }
                    
                    if (alien.getEnemyType() == "nonSpace") {
                        player.setScore(player.getScore() - 10);
                        if (player.getScore() < 0) {
                            player.setScore(0);
                        }
                    }
                    enemyIterator.remove();
                    PlayerBulletList.remove(j);
                    
                    break;
                }
            }
        }
    }

I have 2 lists in a 2D game, Player bullet list and enemy list, and I want to check if they have collided and remove them from the list if they do. Removing a player bullet is fine, but somehow removing an alien causes a crash

I have tried try catch and outputting to console, but it does not seem to display anything after the crash. I suspect a ConcurrentModification Error but I dont know where, as EnemyList is encapsulated within EntityManager. I was expecting both bullet and alien to disappear from the screen on collide, but removing the alien seems to crash the game.

Answer 1

Fixed by putting spawnEnemy in show instead of render

@Override
public void show() {
    AssetsManager.queueLevelMusic();
    AssetsManager.getManager().finishLoading();
    playingSong = AssetsManager.getManager().get(AssetsManager.playingSongPath);

    playingSong.setVolume(audioSettings.getAudioVolume());
    playingSong.setLooping(true);
    if (audioSettings.isAudioEnabled()) {
        playingSong.play();
    }
        
    // On create
    Gdx.input.setInputProcessor(new InputAdapter() {
        
        // Return to menu upon pressing Escape button
        @Override
        public boolean keyDown(int keyCode) {
            if (keyCode == Input.Keys.ESCAPE) {
                game.setScreen(new PauseScene(game));
            }
            return true;
        }
    });
        
    entityManager.spawnEnemy();
}

It kept modifying the list in render when I was trying to remove it on collide.