Unity C# 由于GameController的变量scoreText尚未分配的报错问题

Question

环境：
Unity 2017.3.0f3
window 10
visual Studio 15.5.3
问题描述
官方案例：Space Shooter tutorial，代码无法正常运行？

代码：

using UnityEngine;
using System.Collections;

public class GameController : MonoBehaviour
{
    public GameObject hazard;
    public Vector3 spawnValues;
    public int hazardCount;
    public float spawnWait;
    public float startWait;
    public float waveWait;

    public GUIText scoreText;
    private int score;

    void Start()
    {
        score = 0;
        UpdateScore();
        StartCoroutine(SpawnWaves());
    }

    IEnumerator SpawnWaves()
    {
        yield return new WaitForSeconds(startWait);
        while (true)
        {
            for (int i = 0; i < hazardCount; i++)
            {
                Vector3 spawnPosition = new Vector3(Random.Range(-spawnValues.x, spawnValues.x), spawnValues.y, spawnValues.z);
                Quaternion spawnRotation = Quaternion.identity;
                Instantiate(hazard, spawnPosition, spawnRotation);
                yield return new WaitForSeconds(spawnWait);
            }
            yield return new WaitForSeconds(waveWait);
        }
    }

    public void AddScore(int newScoreValue)
    {
        score += newScoreValue;
        UpdateScore();
    }

    void UpdateScore()
    {
        scoreText.text = "Score: " + score; //问题定位于此处
    }
}