Array or for loop doesn't work with state machine in Unity, gives NullReferenceException

The randomizeMonster function gives a NullReferenceException: Object reference not set to an instance of an object, when it's called from the state machine. randomizeMonster works if i call it from the start or update functions. I think i've tracked the problem to the AllMonsters array, but i don't know how to solve it.

Help is much appreciated :)

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
  
  
  
 public class BattleStateStart : MonoBehaviour {
  
         public string region; //current region
        
  
         public monster[] AllMonsters; //array for premade monsters
         public monster enemyMonster; //the generated enemy
         public monster monsterEquiped; //our monster
  
         // Use this for initialization
         void Start () {
                 Debug.Log(AllMonsters.Length);
                 randomizeMonster ();    //<<<<<<<< Calling randomizeMonster in update or start works, from boths classes
         }
        
         // Update is called once per frame
         void Update () {
                
  
         }
  
         public void PrepareBattle(){
                 Debug.Log ("Preparing for battle");            
                 randomizeMonster ();   
  
  
         }
         //The NullReferenceException: Object reference not set to an instance of an object is somehow caused by the
         //AllMonsters array. If i change AllMonsters.Length bellow to ex. 3 then it works but then the problem jumps to the next                       
         //place AllMonsters[] is used.
         void randomizeMonster(){
                 List<monster> tempMonster = new List<monster>();
                 int randomNumber = Random.Range(0,100);
                
                 if(randomNumber==20){
                         for(int i=0;i<AllMonsters.Length;i++){
                                 if(AllMonsters[i].rarity==monster.Rarity.Rare && AllMonsters[i].Location==region){
                                         tempMonster.Add(AllMonsters[i]);
                                 }
                         }
                 }else{
                         for(int j=0;j<AllMonsters.Length;j++){
                                 if(AllMonsters[j].rarity==monster.Rarity.Common && AllMonsters[j].Location==region){
                                         tempMonster.Add(AllMonsters[j]);
                                 }
                         }
                 }
                
                 int newRandom = Random.Range (0, tempMonster.Count);
                 enemyMonster = tempMonster [newRandom];
                
                
         }
         //Basic monster GUI
         void OnGUI(){
                 //Enemy monster
                 GUI.Label (new Rect (Screen.width / 2, 100, 200, 100), "" + enemyMonster.Name);
                 GUI.Label (new Rect (Screen.width / 2, 110, 200, 100), "HP " + enemyMonster.curHP + "/" + enemyMonster.baseHP);
                        
                 //Player monster
                 GUI.Label (new Rect (50, 100, 200, 100), "" + monsterEquiped.Name);
                 GUI.Label (new Rect (50, 110, 200, 100), "HP " + monsterEquiped.curHP + "/" + monsterEquiped.baseHP);
         }
  
  
 }
  

 using UnityEngine;
 using System.Collections;
  
 public class TurnBasedCombatStateMachine : MonoBehaviour {
  
         private BattleStateStart battleStateStartScript = new BattleStateStart();
  
         //The different states
         public enum Battlestates{      
                 START,
                 PLAYERCHOICE,
                 ENEMYCHOICE,
                 LOSE,
                 WIN,
                 CALCULATEDAMAGE,
                 ADDSTATUSEFFECTS
         }
  
         private Battlestates currentState;
  
  
  
  
         // Use this for initialization
         void Start () {
                 currentState = Battlestates.START;
  
         }
        
         // Update is called once per frame
         void Update () {
                 Debug.Log (currentState);
                 switch (currentState) {
                 case(Battlestates.START):
                         battleStateStartScript.PrepareBattle(); //<<<<< Calling randomizeMonster by calling PreparBattle
                                                                 //      doesn't work, gives
                                                                 //      NullReferenceException: Object reference
                                                                 //      not set to an instance of an object
                         break;
                 case(Battlestates.PLAYERCHOICE):
                         break;
                 case(Battlestates.ENEMYCHOICE):
                         break;
                 case(Battlestates.CALCULATEDAMAGE):
                         break;
                 case(Battlestates.ADDSTATUSEFFECTS):
                         break;
  
  
  
                 case(Battlestates.LOSE):
                         break;
                 case(Battlestates.WIN):
                         //Add XP
                         break;
                 }
         }
  
  
         //Change state button
         void OnGUI(){
                 if(GUILayout.Button("Next State")){
                         if(currentState==Battlestates.START){
                                 currentState=Battlestates.PLAYERCHOICE;
                         }else if(currentState==Battlestates.PLAYERCHOICE){
                                 currentState=Battlestates.ENEMYCHOICE;
                         }else if(currentState==Battlestates.ENEMYCHOICE){
                                 currentState=Battlestates.LOSE;
                         }else if(currentState==Battlestates.LOSE){
                                 currentState=Battlestates.WIN;
                         }else if(currentState==Battlestates.WIN){
                                 currentState=Battlestates.START;
                         }
                 }
  
         }
 }