ToolBox singleton nullreferenceexception on reloading scene

Hi , i m trying singleton for the first time and i cant get rid of the null ref exception on reloading of the scene , i saw post where people get already destroyed problem with application quit , but it s not the same i get

so i have the toolbox setup

 using UnityEngine;
 using System.Collections;
 
 public class Toolbox : Singleton<Toolbox> {
     protected Toolbox () {} // guarantee this will be always a singleton only - can't use the constructor!
  
     //public string myGlobalVar = "whatever";
      public bool allRaycastDebugOn = false;
      public bool allRaycastDepth = false;
      public bool RandomSeed = false;
      public string seed ;
     //public Language language = new Language();
  
     void Awake () {
         // Your initialization code here
 
     }
  
     // (optional) allow runtime registration of global objects
     static public T RegisterComponent<T> () where T: Component {
         return Instance.GetOrAddComponent<T>();
     }
 }

the singleton

 using UnityEngine;
 using System.Collections;
  
 /// <summary>
 /// Be aware this will not prevent a non singleton constructor
 ///   such as `T myT = new T();`
 /// To prevent that, add `protected T () {}` to your singleton class.
 /// 
 /// As a note, this is made as MonoBehaviour because we need Coroutines.
 /// </summary>
 public class Singleton<T> : MonoBehaviour where T : MonoBehaviour
 {
     private static T _instance;
  
     private static object _lock = new object();
  
     public static T Instance
     {
         get
         {
             if (applicationIsQuitting) {
                 Debug.LogWarning("[Singleton] Instance '"+ typeof(T) +
                     "' already destroyed on application quit." +
                     " Won't create again - returning null.");
                 return null;
             }
  
             lock(_lock)
             {
                 if (_instance == null)
                 {
                     _instance = (T) FindObjectOfType(typeof(T));
  
                     if ( FindObjectsOfType(typeof(T)).Length > 1 )
                     {
                         Debug.LogError("[Singleton] Something went really wrong " +
                             " - there should never be more than 1 singleton!" +
                             " Reopenning the scene might fix it.");
                         return _instance;
                     }
  
                     if (_instance == null)
                     {
                         GameObject singleton = new GameObject();
                         _instance = singleton.AddComponent<T>();
                         singleton.name = "(singleton) "+ typeof(T).ToString();
  
 //                        DontDestroyOnLoad(singleton);
 // 
                         Debug.Log("[Singleton] An instance of " + typeof(T) + 
                             " is needed in the scene, so '" + singleton +
                             "' was created with DontDestroyOnLoad.");
                     } else {
 //                        DontDestroyOnLoad(_instance.gameObject);
 //                        
                         Debug.Log("[Singleton] Using instance already created: " +
                             _instance.gameObject.name);
                     }
                 }
  
                 return _instance;
             }
         }
     }
  
     private static bool applicationIsQuitting = false;
     /// <summary>
     /// When Unity quits, it destroys objects in a random order.
     /// In principle, a Singleton is only destroyed when application quits.
     /// If any script calls Instance after it have been destroyed, 
     ///   it will create a buggy ghost object that will stay on the Editor scene
     ///   even after stopping playing the Application. Really bad!
     /// So, this was made to be sure we're not creating that buggy ghost object.
     /// </summary>
     public void OnDestroy () {
         applicationIsQuitting = true;
     }
 }

the methodeextention

 using UnityEngine;
 using System.Collections;
 
 static public class MethodExtensionForMonoBehaviourTransform {
     /// <summary>
     /// Gets or add a component. Usage example:
     /// BoxCollider boxCollider = transform.GetOrAddComponent<BoxCollider>();
     /// </summary>
     static public T GetOrAddComponent<T> (this Component child) where T: Component {
         T result = child.GetComponent<T>();
         if (result == null) {
             result = child.gameObject.AddComponent<T>();
         }
         return result;
     }
 }

first script to get the seed string from my toolbox

 using UnityEngine;
 using System.Collections;
 
 public class ToolBoxTester : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
         //Toolbox toolbox = Toolbox.Instance;
         print("seed = " + Toolbox.Instance.seed);
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }
 

and a script that restart the scene on keypress

 using UnityEngine;
 using System.Collections;
 
 public class RestartGame : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         if(Input.GetKeyDown(KeyCode.N)){UnityEngine.SceneManagement.SceneManager.LoadScene(PlayerPrefs.GetInt("currentscenesave"));}//Application.LoadLevel(0);}
     }
 }

at start everything is fine , i get my seed printed out but ; when i press the reloadmyscene button i get the error

 NullReferenceException: Object reference not set to an instance of an object
 ToolBoxTester.Start () (at Assets/Scripts/ToolBoxTester.cs:9)
 

 

my question is , what should i do to not get this error

thanks

[EDIT]

i have the feeling that my problem is related to if (applicationIsQuitting) {return null;} in the singleton ; i m tempted to cut it out of the code , but then i dont know how i would be able to detect if i have a buggy ghost object in my scene .

[/EDIT]