代码中的Unity Oculus gearVr自动行走错误

Question

伙计们，我正在Unity中为Gear Vr开发一个Vr App。我写了这段代码：

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

public class controller : MonoBehaviour {
    public GameObject ground;
    private bool walking= false;
    private Vector3 spawnPoint;


    // Use this for initialization
    void Start () {
        spawnPoint = transform.position;
    }

    // Update is called once per frame
    void Update () {
        if (walking){
            transform.position = transform.position + Camera.main.transform.forward * .5f * Time.deltaTime;
        }

        if (transform.position.y<-10f){
            transform.position = spawnPoint;

        }
         //The compiler is giving the error here
        Ray ray = Camera.main.ViewportPointToRay(new Vector3(.5f, .5f,0)); 
         //^
        RaycastHit hit;

        if(Physics.Raycast(ray,out hit)){
            walking = false;
        } else{
            walking = true;
        }



    }
}

运行代码后，出现以下错误： NullReferenceException：对象引用未设置为对象的实例 controller.Update（）（位于Assets / controller.cs：26） Virtual Reality SDK Oculus初始化失败。

感谢ZZaim