Null reference exception error in Unity, Please Help!

I have created a script earlier today and in mono develop it showed no errors but when I tested it it gave me the error: NullReferenceException: Object reference not set to an instance of an object CharacterGenerator.DisplayVitals() CharacterGenerator.OnGUI() Here is my script which is producing the error:

using UnityEngine; using System.Collections; using System;

public class CharacterGenerator : MonoBehaviour { private PlayerCharacter _toon;

 // Use this for initialization
 void Start () {
     _toon = new PlayerCharacter();
     _toon.Awake();
 }
 
 // Update is called once per frame
 void Update () {
 
 }
 
 void OnGUI() {
     DisplayName ();
     DisplayAttributes();        
     DisplayVitals();
     DisplaySkills();
 }
 
 private void DisplayName() {
     GUI.Label (new Rect(10, 10, 50, 25), "Name:");
     _toon.Name = GUI.TextArea(new Rect(65, 10, 100,25), _toon.Name);
 }
 
 private void DisplayAttributes() {
     for(int cnt = 0; cnt < Enum.GetValues(typeof(AttributeName)).Length; cnt++) {
         GUI.Label(new Rect(10, 40 + (cnt * 25), 100, 25), ((AttributeName)cnt).ToString());
         GUI.Label(new Rect(115, 40 + (cnt * 25), 30, 25), _toon.GetPrimaryAttribute(cnt).AdjustedBaseValue.ToString());
     }
 }
 
 private void DisplayVitals() {
     for(int cnt = 0; cnt < Enum.GetValues(typeof(VitalName)).Length; cnt++) {
         GUI.Label(new Rect(10, 40 + ((cnt + 7) * 25), 100, 25), ((VitalName)cnt).ToString());
         GUI.Label(new Rect(115, 40 + ((cnt + 7) * 25), 30, 25), _toon.GetVital(cnt).AdjustedBaseValue.ToString());
     }
 }
 
 private void DisplaySkills() {
     for(int cnt = 0; cnt < Enum.GetValues(typeof(SkillName)).Length; cnt++) {
         GUI.Label(new Rect(250, 40 + (cnt * 25), 100, 25), ((SkillName)cnt).ToString());
         GUI.Label(new Rect(355, 40 + (cnt * 25), 30, 25), _toon.GetSkill(cnt).AdjustedBaseValue.ToString());
     }
 }

}