Unity 4.5 to 5 problem

Hello, i watch one tutorial to create a 2D Game and i think that i have problem with the different versions of Unity or ? maybe something else. The first Screen is the tutorial and the second my Code. I noticed that the numbers inside the string appeared red but in tutorial appeared green. i thought that maybe this is due to the older version of unity. But when i pressed to play i had this error.

NullReferenceException: Object reference not set to an instance of an object GameHud.OnGUI () (at Assets/Code/GameHud.cs:15)

Anyone who can help please ! i am confused !

here is and my code :

using UnityEngine;

public class GameHud : MonoBehaviour { public GUISkin Skin;

 public void OnGUI()
 {
     GUI.skin = Skin;

     GUILayout.BeginArea(new Rect(0, 0, Screen.width, Screen.height));
     {
         GUILayout.BeginVertical(Skin.GetStyle("GameHud"));
         {
             GUILayout.Label(string.Format("Points: {0}", GameManager.Instance.Points), Skin.GetStyle("PointsText"));

             var time = LevelManager.Instance.RunningTime;
             GUILayout.Label(string.Format(
                 "{0:00}:{1:00} with {2} bonus",
                 time.Minutes + (time.Hours * 60),
                 time.Seconds,
                 LevelManager.Instance.CurrentTimeBonus), Skin.GetStyle("TimeText"));
         }
         GUILayout.EndVertical();
     }
     GUILayout.EndArea(); 
 }

}