void Awake()
 {
     if (instance == null) {
                     instance = this;
                     DontDestroyOnLoad (this);
             } else {
                     DestroyImmediate (gameObject);//destroy this gameobject if it's already exist
             }
 }
 void Awake()
 {
     if (instance == null)
     {
         instance = this;
         DontDestroyOnLoad(this);
     }
     else
     {
         DestroyImmediate(gameObject);                                     //destroy this gameobject if it's already exist
     }
 }