There's a possibility of java.lang.NullPointerException when trying to open a GUI

[wtlgo]

The plugin can cause a java.lang.NullPointerException when a player is trying to open a GUI.

In PlayerManager.findPlayer method there is a possibility that the method returns null if createError != null

	public SkullUser findPlayer(@NonNull final UUID uuid) {
		final AtomicReference<SkullUser> skullUser = new AtomicReference<>(this.players.getOrDefault(uuid, null));

		if (skullUser.get() != null) return skullUser.get();

		Skulls.getDataManager().insertPlayer(new SkullPlayer(uuid, new ArrayList<>()), (createError, created) -> {
                        // Oh no, an error has happened....
			if (createError == null) { // That's why we are skipping this part...
				addPlayer(created);
				skullUser.set(created);
			}

                        // error is not handled in any meaningful way, the user is not aware that something wrong has happened...
		});

               // That's why we return null...
		return skullUser.get();
	}

This messes up with SkullsViewGUI constructor, because the result of PlayerManager.findPlayer is passed there as is at many places, and it will throw a java.lang.NullPointerException exception

	public SkullsViewGUI(final Gui parent, final SkullUser skullPlayer /* == null */, final String category, final ViewMode viewMode) {
                /* ... */
		this.skullPlayer = skullPlayer;
		this.player = Bukkit.getPlayer(this.skullPlayer.getUUID()); // <- Oops!
                // A NullPointerException happened and is not caught, now it is all over the logs, the user is confused...
		draw();
	}

I can't really suggest a good way to fix this, since I'm not fully familiar with the code base yet, but I think you would like to be aware of this problem.