空引用异常：对象未设置为对象的实例

Question

完成Zig Zag游戏后，我正在使用Unity 2018.2.18f1。当我单击启动时，会出现一条错误消息，显示NullReferenceException：对象未设置为对象的实例。它总是在GameManager脚本中发生。你能帮我吗？

GameManager Javascript的代码：

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class GameManager : MonoBehaviour {

    public static GameManager instance { get; private set; }
    public bool gameOver;

    void Awake()
    {
        if(instance == null)
        {
            instance = this;
        }
    }
    // Use this for initialization
    void Start () {
        gameOver = false;
    }

    // Update is called once per frame
    void Update () {

    }

    public void StartGame()
    {
        UIManager.instance.GameStart();
        ScoreManager.instance.startScore();
        GameObject.Find("PlatformSpawner").GetComponent <PlatformSpawner>().StartSpawningPlatforms();
    }

    public void GameOver()
    {
        UIManager.instance.GameOver();
        ScoreManager.instance.stopScore();
        gameOver = true;
    }
}