Unity Oculus gearVr Auto walking error in code

Hey guys I am developing a Vr App in Unity for gear Vr. I am using Unity 2018.2.0f2 (64bit)  on mac. This is  My Oculus SDK: oculussig_ce12160c34e98e2301 . I wrote this code:

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;
        }



    }
}

After I run the code I get these Errors: NullReferenceException: Object reference not set to an instance of an object controller.Update () (at Assets/controller.cs:26) Virtual Reality SDK Oculus failed to initialize.

Thanks in advance ZZaim