在代码（https://livebook.manning.com/book/unity-in-action-third-edition/chapter-3/166）中，添加火球攻击敌人的能力时，文件被编译，当您启动游戏并遇到敌人时，出现错误：

UnassignedReferenceException: The variable fireballPrefab of WanderingAI has not been assigned.
You probably need to assign the fireballPrefab variable of the WanderingAI script in the inspector.
UnityEngine.Object.Instantiate[T] (T original) (at <e8a406da998549af9a2680936c7da25a>:0)
WanderingAI.Update () (at Assets/WanderingAI.cs:31)

对象已添加https://prnt.sc/hudEm5hvcuWx

void Update()
    {   
        if (_alive) 
        {
            transform.Translate(0, 0, speed * Time.deltaTime);
            Ray ray = new Ray(transform.position, transform.forward); // непрерывное движение
            RaycastHit hit;
            if (Physics.SphereCast(ray, 0.75f, out hit)) // бросаем луч с описанной вокруг него окружностью
        {   GameObject hitObject = hit.transform.gameObject;
            if (hitObject.GetComponent<PlayerCharacter>()) 
            {
                if (_fireball == null) 
                {
                    _fireball = Instantiate(fireballPrefab) as GameObject;
                    _fireball.transform.position = transform.TransformPoint(Vector3.forward * 1.5f);
                    _fireball.transform.rotation = transform.rotation;
                }
            }
            else if (hit.distance < obstacleRange) 
            {
                float angle = Random.Range (-110, 110); // поворот с наполовину случайным выбором направления
                transform.Rotate(0, angle, 0);
            }
        }
        }
    }
}