学习《Unity2018入门与实战》中易出现的错误_unity text legacy

1.执行第4章中4.6节创建切换UI的调度器的图List 4-3代码时，易报错：“NullReferenceException: Object reference not set to an instance of an object”（P147）

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;  //使用UI组件时要先引入它
 
public class GameDirector : MonoBehaviour{
    GameObject car;
    GameObject flag;
    GameObject distance;  
 
    void Start(){
        this.car = GameObject.Find("car");  
        this.flag = GameObject.Find("flag");
        this.distance = GameObject.Find("Distance");
}
 
 
    void Update(){
        float length = this.flag.transform.position.x - this.car.transform.position.x;  
        this.distance.GetComponent<Text>().text = "距离目标" + length.ToString("F2") + "m";
}
}

List 4-3 "显示距离信息"的脚本  

如果在P142创建UI时