为什么我得到一个空引用异常？标记正确。

Question

我正在尝试做一些简单的2D光线投射，我完全迷失了。我在以下脚本的第25行获得了一个空引用异常。我确保敌人被标记为敌人，然而当我尝试获取标签时，我得到了错误。我现在意识到错误只发生在我点击天空时，而不是在没有标记的游戏对象上。

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

public class Shoot : MonoBehaviour {
    public Camera cam;

    public GameObject enemy;

    public Transform fireStart;

    GameObject target;

    // Use this for initialization
    void Start () {
        float inputX = Input.GetAxisRaw("Mouse X");
        float inputY = Input.GetAxisRaw("Mouse Y");
        fireStart = transform.FindChild("GunEnd");
    }
    void ShootGun()
    {
        Debug.Log("Fire");
        Vector2 mousePos = new Vector2(Camera.main.ScreenToWorldPoint(Input.mousePosition).x, 
            Camera.main.ScreenToWorldPoint(Input.mousePosition).y);
        Vector2 firePointPos = new Vector2(fireStart.position.x, fireStart.position.y);
        RaycastHit2D hit = Physics2D.Raycast(firePointPos, mousePos - firePointPos, 100f);
        if (hit.collider.CompareTag("Enemy"))
        {
            Debug.Log("Hit");
        }
        else if (hit.collider.CompareTag("Untagged"))
        {
            Debug.Log("No hit");
        }
    }
    // Update is called once per frame
    void Update () {

        if (Input.GetButtonDown("Fire1"))
        {
            ShootGun();
        }
    }

}

非常感谢