NullReferenceException: Object Reference Not Set To An Instance Of An Object

So I have a script that I have put onto a weapon, which is parented to the player's hand. The player game object has a script called PlayerClass that has the list of stats for te player. when the weapon hits an enemy, the weapon script is called:

 using UnityEngine;
 using System.Collections;
 
 public class WeaponAttackScript : MonoBehaviour 
 {
     public delegate void WeaponHit ();  /*this is necessary for the event*/
     public static event WeaponHit OnHit;  /*sets up the event so that we know when the weapon hits an enemy*/
 
     // Use this for initialization
     void Start () 
     {
     
     }
     
     // Update is called once per frame
     void Update () 
     {
 
     }
 
     void OnTriggerEnter(Collider enemyCollider)   /*trigger allows us to avoid the terrain-flying bug.*/
     {
         if(enemyCollider.gameObject.tag=="Enemy")
 
         {
             PlayerClass playerStats = this.gameObject.transform.parent.GetComponent<PlayerClass>(); /*for getting player's strength*/
             InventoryItemClass weaponType = GetComponent<InventoryItemClass>();/*the item class, needed for
             getting the base damage*/
             EnemyTypeClass enemyType = enemyCollider.gameObject.GetComponent<EnemyTypeClass>(); /*references
             enemy's type to get armour rating (see next line)*/
             float enemyArmourRating = enemyType.armourRating; /*self-explanatory*/
             float baseDamage =  weaponType.itemDamage;        /*"*/
             float attackerStrength = playerStats.strength;    /*"*/
             float damageDealt = (baseDamage/enemyArmourRating)*(attackerStrength/100.0f);
             if (PlayerClass.isAttacking == true)  /*just need to sort out the double-hit issue*/
                 
             {
                 enemyType.health = enemyType.health - damageDealt;
                 OnHit ();
                 Debug.Log ("Hit for " + damageDealt + " damage");
                 
             }
         }
 
     }
 }
         

but when I hit the enemy, this error message pops up:

NullReferenceException: Object reference not set to an instance of an object WeaponAttackScript.OnTriggerEnter (UnityEngine.Collider enemyCollider) (at Assets/scripts/CombatScripts/WeaponAttackScript.cs:33)

where have I gone wrong?

Edit: I fixed it by using GetComponentInParent instead of this.gameObject.transform.parent.GetComponent, but I still wouldn't mind knowing why the previous method didn't work.