BossBar#viewers always return an empty list

[GoodestEnglish]

Is there an existing issue for this?

• [X] I have searched the existing issues

Are you using the latest version of MockBukkit?

• [X] I am using the latest version of MockBukkit.

Minecraft Version

1.20

Describe the bug

So if you try to use BossBar#viewers to loop the current viewers and remove viewers from the BossBar, it doesn't produce the excepted result.

Reproducible Test

Just try to use this code to test, and you will be able to reproduce the unexcepted result.

    @Test
    void test() {
        PlayerMock player = server.addPlayer();
        BossBar bossBar = BossBar.bossBar(Component.text("hello world"),1, BossBar.Color.RED, BossBar.Overlay.PROGRESS);

        bossBar.addViewer(player);

        assertTrue(player.getBossBars().contains(bossBar)); //Passed :D

        bossBar.viewers().forEach(viewer -> {
            if (viewer instanceof Player p) {
                bossBar.removeViewer(p);
            }
        });

        assertFalse(player.getBossBars().contains(bossBar)); //Failed :(
    }

Anything else?

No response

[Thorinwasher]

I can reproduce this.

We need to register a service for this to work properly