Null Reference Exception 에러

참조할 수 없는 오브젝트를 참조하려고 할때 해당 에러가 나온다고 하는데, 지금 InputManager.cs ,Managers.cs , PlayerController.cs 전부 코드상 강의와 똑같고 문제가 없습니다. 무엇이 문제일까요

아래는 코드와 에러메세지입니다. 

Managers.cs에서 10번째 라인,PlayerController 17번째 라인에서 문제가 생기는데

 public static InputManager Input { get { return Instance._input; } }

        Managers.Input.KeyAction -= OnKeyboard;

        Managers.Input.KeyAction += OnKeyboard;

이부분이네요. 

/////////// InputManager

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

public class InputManager
{
    public Action KeyAction = null;

    public void OnUpdate()
    {
        if (Input.anyKey == false)
            return;
        if (KeyAction != null)
            KeyAction.Invoke();
    }
}
//////////////////// Managers

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

public class Managers : MonoBehaviour
{
    static Managers s_instance;
    static Managers Instance { get { Init(); return s_instance; } }
    InputManager _input = new InputManager();
    public static InputManager Input { get { return Instance._input; } }
    void Start()
    {
        Init();
    }


    void Update()
    {
        _input.OnUpdate();
    }
    static void Init()
    {
        if(s_instance == null)
        {
            GameObject go = GameObject.Find("@Managers");
            if(go == null)
            {
                go = new GameObject { name = "@Managers" };
                go.AddComponent<Managers>();
            }
            DontDestroyOnLoad(go);
            Managers mg = go.GetComponent<Managers>();
        }

    }
}
///////////////////////// PlayerController

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

public class PlayerController : MonoBehaviour
{
    [SerializeField]
    float _speed = 10.0f;
    void Start()
    {
        Managers.Input.KeyAction -= OnKeyboard;
        Managers.Input.KeyAction += OnKeyboard;
    }


    void Update()
    {

    }
    void OnKeyboard()
    {
        if (Input.GetKey(KeyCode.W))
        {
            transform.rotation = Quaternion.Slerp(transform.rotation, Quaternion.LookRotation(Vector3.forward), 0.2f);
            transform.position += Vector3.forward * Time.deltaTime * _speed;
        }
        if (Input.GetKey(KeyCode.S))
        {
            transform.rotation = Quaternion.Slerp(transform.rotation, Quaternion.LookRotation(Vector3.back), 0.2f);
            transform.position += Vector3.back * Time.deltaTime * _speed;
        }
        if (Input.GetKey(KeyCode.A))
        {
            transform.rotation = Quaternion.Slerp(transform.rotation, Quaternion.LookRotation(Vector3.left), 0.2f);
            transform.position += Vector3.left * Time.deltaTime * _speed;
        }
        if (Input.GetKey(KeyCode.D))
        {
            transform.rotation = Quaternion.Slerp(transform.rotation, Quaternion.LookRotation(Vector3.right), 0.2f);
            transform.position += Vector3.right * Time.deltaTime * _speed;
        }
    }
}