Why do I keep getting this error?

Questions & Answers
1

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.

2

After rechecking my tags to verify, I realized that the object that has the attackTrigger component attached to it is neither child nor a parent. It is a projectile that I spawn in another script. The attackTrigger script is just to clarify the damage that the spawned object will deal. How would I call the script on the object?