NullReferenceException: Object reference not set to an instance of an object (XmlDocument.LoadXml())

Hello! I have a problem with XML file when I press Start button on Unity:

NullReferenceException: Object reference not set to an instance of an object AbilitiesExtractor.extraerAbs () (at Assets/Scripts/Abilities/AbilitiesExtractor.cs:22) BattleGUI.Start () (at Assets/Scripts/TurnBasedCombat/BattleGUI.cs:19)

NullReferenceException: Object reference not set to an instance of an object BattleGUI.OnGUI () (at Assets/Scripts/TurnBasedCombat/BattleGUI.cs:30) UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Here are the classes:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using System.Xml;
 using System.IO;
 
 public class AbilitiesExtractor : MonoBehaviour
 {
     public TextAsset abilityList;
     public static List<Ability> abilities = new List<Ability>();
     public List<Ability> extraerAbs()
     {
         XmlDocument xmlDocument = new XmlDocument();
         xmlDocument.LoadXml(abilityList.text);
         XmlNodeList itemList = xmlDocument.GetElementsByTagName("Ability");
         Debug.Log("Lista de habilidades: "+itemList.Count);
         for (int i = 0; i < itemList.Count; i++)
         {
             Ability temp = new Ability();
             temp.RequiredWeapon = itemList[i].SelectSingleNode("RequiredWeapon").InnerText;
             temp.RequiredSkill = itemList[i].SelectSingleNode("RequiredSkill").InnerText;
             temp.Range = int.Parse(itemList[i].SelectSingleNode("Range").InnerText);
             temp.Name = itemList[i].SelectSingleNode("Name").InnerText;
             temp.Description = itemList[i].SelectSingleNode("Description").InnerText;
             temp.Target = int.Parse(itemList[i].SelectSingleNode("Target").InnerText);
             temp.Power = int.Parse(itemList[i].SelectSingleNode("Power").InnerText);
             abilities.Add(temp);
         }
         return abilities;
     }
 }

And the second one:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class BattleGUI : MonoBehaviour
 {
     private string playerName;
     private int playerLevel;
     private int playerTenacity;
     private List<Ability> abilities;
     AbilitiesExtractor absExtractor = new AbilitiesExtractor();
 
 
     void Start()
     {
         playerName = GameInformation.PlayerName;
         playerLevel = GameInformation.PlayerLevel;
         playerTenacity = GameInformation.Tenacity;
         abilities = absExtractor.extraerAbs();
     }
 
 
     void Update()
     {
         
     }
 
     void OnGUI()
     {
         GUI.Button(new Rect(0, 0, 0, 0), abilities[1].Name);
     }
 }

I have both of them attached to MainCamera, and the XML attached to AbilitiesExtractor. Here is the XML:

 <AbilityList>
     <Ability>
         <RequiredWeapon>Unarmed</RequiredWeapon>
         <RequiredSkill level="0">MartialArts</RequiredSkill>
         <Range>1</Range>
         <Name>Kick</Name>
         <Description>Kick that trash!</Description>
         <Target>1</Target>
         <Power>12</Power>
     </Ability>
     <Ability>
         <RequiredWeapon>Handgun</RequiredWeapon>
         <RequiredSkill level="0">Firearms</RequiredSkill>
         <Range>1</Range>
         <Name>Shot</Name>
         <Description>Shot that trash!</Description>
         <Target>1</Target>
         <Power>10</Power>
     </Ability>
 </AbilityList>

It is my first time working with XML on unity, but I have other clases working fine with other XML (objects), and I can't see the failure here. I will be very gratefull for any help you can give to me.

EDIT: If I test it on Awake method on the AbilitiesExtractor class (BattleGui removed), it run correctly. So the problem is calling the class, because class1(caller) can't reach xml attached to class2(called)? I don't know how it works. Here is the Awake method:

     void Awake()
     {
         extraerAbs();
         foreach(Ability ab in abilities)
         {
             Debug.Log(ab.Name + "\n" + ab.Description);
         }
     }