How do I fix this error

The error is called :NullReferenceException: Object reference not set to an instance of an object CharacterStat.AddStatBonus (System.Collections.Generic.List`1 statBonuses) (at Assets/scripts/weapon/CharacterStat.cs:17) PlayerWeaponController.EquipWeapon (.Item ItemToEquip) (at Assets/scripts/weapon/PlayerWeaponController.cs:30) InventoryController.Update () (at Assets/scripts/weapon/InventoryController.cs:21)

 using System.Collections.Generic;
 using UnityEngine;
 
 public class CharacterStat : MonoBehaviour
 {
     public List<Basestat> stats = new List<Basestat>();
 void Start()
 {
     stats.Add(new Basestat(5, "power", "your power level."));
     stats.Add(new Basestat(3, "vittality", "your vitality level"));
 }
 public void AddStatBonus(List<Basestat> statBonuses)
 {
     foreach (Basestat statBonus in statBonuses)
         stats.Find(x => x.StatName == statBonus.StatName).AddStatBonus(new StatBonus(statBonus.BaseValue));

 }



 public void RemoveStatBonus(List<Basestat> statBonuses)
 {
     foreach (Basestat statBonus in statBonuses)
     {
         stats.Find(x => x.StatName == statBonus.StatName).RemoveStatBonus(new StatBonus(statBonus.BaseValue));
     }
 }

}

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerWeaponController : MonoBehaviour {
 
     public GameObject PlayerHand;
     public GameObject EquipedWeapon { get; set; }
     CharacterStat characterstats;
 
     IWeapon equipedWeapon;
 
      void Start()
     {
         characterstats = GetComponent<CharacterStat>();
     }
 
     public void EquipWeapon(Item ItemToEquip)
     {
         if (EquipedWeapon != null)
         {
             characterstats.RemoveStatBonus(EquipedWeapon.GetComponent<IWeapon>().Stats);
             Destroy(PlayerHand.transform.GetChild(0).gameObject);
         }
 
         EquipedWeapon = (GameObject)Instantiate(Resources.Load<GameObject>("Weapons/" + ItemToEquip.ObjectSlug), PlayerHand.transform.position, PlayerHand.transform.rotation);
         equipedWeapon =  EquipedWeapon.GetComponent<IWeapon>();
 
         EquipedWeapon.transform.SetParent(PlayerHand.transform);
         characterstats.AddStatBonus(ItemToEquip.Stats);
         Debug.Log(equipedWeapon.Stats[0].GetCalculatedStatValue());
     }
     public void PerformWeaponAttack()
     {
         equipedWeapon.PerformAttack();
     }
 }
 -------------------------------------------------------------------------------------------------------------------------------------------------------
 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class InventoryController : MonoBehaviour {
     public PlayerWeaponController playerWeaponController;
     public Item sword;
 
     void Start()
     {
         playerWeaponController = GetComponent<PlayerWeaponController>();
 
         List<Basestat> swordStats = new List<Basestat>();
         swordStats.Add(new Basestat(6, "Power", "your power level"));
         sword = new Item(swordStats, "sword");
     }
     void Update()
     {
        if(Input.GetKeyDown(KeyCode.V))
         {
             playerWeaponController.EquipWeapon(sword);
         }
 
     }
 }