对象引用未设置为对象的实例......但它确实如此

Question

所以我在这里有一些代码：

public class World
{
    private static World _world = null;
    public static World world
    {
        get
        {
            if (_world == null)
                _world = new World();
            return _world;
        }
    }

    private Enemy[] Monsters = new Enemy[2];
    public Enemy[] monsters
    {
        get { return Monsters; }
    }

    private Sword[] Swords = new Sword[2];
    public Sword[] sword
    {
        get { return Swords; }
    }

    public void PopulateEnemies()
    {
        Enemy Goblin = new Enemy("Goblin", "Looks dirty and not so friendly", 100);
    }      

    public void PopulateItems()
    {
        Sword Excalibur = new Sword("Excalibur", 100);
    }
}

这是我的Main()方法：

static void Main()
{
    World.world.PopulateItems();
    World.world.PopulateEnemies();        
    Player player = new Player("", 50, 50, 1, 0, 100);

    DisplayText("Welcome warrior, what is your name? ");
    player.Name = Console.ReadLine();

    if (player.Name == string.Empty)
    {
        DisplayText("You don't want to tell me your name, all right... I'll call you Norman!");
    }
    else
    {
        DisplayText("Nice to meet you {0}, what are you waiting for, go kill some monsters!", player.Name);
    }
    Console.ReadLine();
    Console.WriteLine();

    DisplayText("You find a nice sword in a cavern. You pick it up!");
    World.world.sword[0].PickUp();
    World.world.monsters[0].EnemyEncounter();
}

现在，当我到达PickUp()方法时，它会抛出异常，但我已经在另一个类中创建了一个新的Sword，并使用PopulateItems来确保我的Main()得到它。现在，我做错了什么？