A problem "NullReferenceException: Object reference not set to an instance of an object(...)"

Hello. I have a problem: "NullReferenceException: Object reference not set to an instance of an object CreateNewCharacter.OnGUI () (at Assets/Scripts/Create Player/CreateNewCharacter.cs:30)". My codes:

CreateNewCharacter:

 using UnityEngine;
 using System.Collections;
 
 public class CreateNewCharacter : MonoBehaviour {
 
  private BasePlayer newPlayer;
  private bool isMageClass;
  private bool isWarriorClass;
 
 
  void Start () {
   newPlayer = new BasePlayer();
  }
 
  void Update () {
  
  }
 
  void OnGUI(){
   isMageClass = GUILayout.Toggle (isMageClass, "Mag");
   isMageClass = GUILayout.Toggle (isWarriorClass, "Wojownik");
   if (GUILayout.Button ("Stwórz")) {
    if (isMageClass) {
     newPlayer.PlayerClass = new BaseMageClass ();
    } else if (isWarriorClass) {
     newPlayer.PlayerClass = new BaseWarriorClass ();
    }
    newPlayer.PlayerLevel = 1;
    newPlayer.Stamina = newPlayer.PlayerClass.Stamina;
    newPlayer.Endurance = newPlayer.PlayerClass.Endurance;
    newPlayer.Intellect = newPlayer.PlayerClass.Intellect;
    newPlayer.Strength = newPlayer.PlayerClass.Strength;
 
    Debug.Log("Klasa postaci: " + newPlayer.PlayerClass.CharacterClassName);
    Debug.Log("Poziom postaci: " + newPlayer.PlayerLevel);
    Debug.Log("Poziom staminy: " + newPlayer.Stamina);
    Debug.Log("Poziom wytrzymałości: " + newPlayer.Endurance);
    Debug.Log("Poziom intelektu: " + newPlayer.Intellect);
    Debug.Log("Poziom siły: " + newPlayer.Strength);
   }
  }
 }

BasePlayer:

 using UnityEngine;
 using System.Collections;
 
 public class BasePlayer {
 
  private string playerName;
  private int playerLevel;
  private BaseCharacterClass playerClass;
  private int stamina;
  private int endurance;
  private int intellect;
  private int strength;
 
  public string PlayerName{
   get{return playerName;}
   set{playerName = value;}
  }
  public int PlayerLevel{
   get{return playerLevel;}
   set{playerLevel = value;}
  }
  public BaseCharacterClass PlayerClass{
   get{return playerClass;}
   set{playerClass = value;}
  }
  public int Stamina{
   get{return stamina;}
   set{stamina = value;}
  }
  public int Endurance{
   get{return endurance;}
   set{endurance = value;}
  }
  public int Intellect{
   get{return intellect;}
   set{intellect = value;}
  }
  public int Strength {
   get{ return strength; }
   set{ strength = value; }
  }
 }

     I have Unity 5. But maybe in the Unity 5 changed some commands ? Please help ^^ :-(


     And i have a problem with private bool isWarriorClass. MonoDevelop is telling me that there isnt any value and this will be 'false'. Its not a red error but orange error: "Assets/Scripts/Create Player/CreateNewCharacter.cs(8,22): warning CS0649: Field `CreateNewCharacter.isWarriorClass' is never assigned to, and will always have its default value `false' " Maybe it's a problem?


     But the tutorial that watch was created in 2014 , so if I know , then it was unity 4. It can either command has been changed in version 5 ?