Why do I keep getting this error?

Hey everyone, I've been using unity for about 8 months now. I'm making a top down dungeon game with a few friends and we've been able to solve every problem except this one. When I pick up my item that is suppose to increase the damage of my weapon. I get this error.

NullReferenceException: Object reference not set to an instance of an object PowerUp.OnTriggerEnter2D (UnityEngine.Collider2D other) (at Assets/Scripts/PowerUp.cs:12)

PowerUp Script (C#);

 //PowerUp Script
 using UnityEngine;
 using System.Collections;
 
 public class PowerUp : MonoBehaviour {
 
     public int addDamage;
 
 
     void OnTriggerEnter2D (Collider2D other)
     {
         if (other.gameObject.tag == "Player") {
             other.gameObject.GetComponent<attackTrigger> ().PowerUpIncrease (addDamage); 
             Destroy (gameObject);
         }
     }
 }

My Attack Trigger/Damage Script (C#):

 using UnityEngine;
 using System.Collections;
 
 public class attackTrigger : MonoBehaviour 
 {
     // Sets how much damage we want to do to the enemy. 
 
     public int startDamage;
     public int currentDamage;
     public int maxDamage;
 
 
     void Start ()
     {
         //Sets Starting Damage
         currentDamage = startDamage;
     }
     void Update()
     {
         //Makes it so damage can't be maxed not over the intended amount
         if (currentDamage>maxDamage){
             currentDamage = maxDamage;
         }
     }
         public void OnTriggerEnter2D(Collider2D other)
         {
             // Checks if the tag is listed as 'Enemy'. 
             if (other.gameObject.tag == "Enemy") 
             {
             // Accesses the enemyDamage.cs script. 
             other.gameObject.GetComponent<enemyDamage> ().HurtEnemy (currentDamage);  
             }
         }
 
         //The called variable in the other class...
     public void PowerUpIncrease(int addDamage){
         //Adds additional damage to the current damage...
         currentDamage += addDamage;
     }
 }

Enemy Damage Script (C#):

 using UnityEngine;
 using System.Collections;
 
 public class enemyDamage : MonoBehaviour 
 {
     public int MaxHealth = 100; 
     public int CurrentHealth; 
 
     void Start ()
     {
         // At the start of our game, we want the current health of the enemy to equal the max health.
         CurrentHealth = MaxHealth; 
     }
 
     void Update ()
     {
         // If the enemy's current health is less than zero...
         if (CurrentHealth <= 0) 
         {
             // Remove it from the game. 
             Destroy (gameObject); 
         }
     }
 
     public void HurtEnemy(int currentDamage)
     {
         CurrentHealth -= currentDamage; 
     }
 
     public void SetMaxHealth()
     {
         CurrentHealth = MaxHealth; 
     }
 
 
 }

The goal that I am trying to reach is when I get my power up. For the damage to increase.