What is wrong with my code

I’m making a script that allows the player to fire bullets at an enemy but when I finished making it I got this error:
NullReferenceException: Object reference not set to an instance of an object
Playershooting.Attack () (at Assets/Scripts/Player Scripts/Playershooting.cs:40)
Playershooting.Update () (at Assets/Scripts/Player Scripts/Playershooting.cs:27)

Here are my scripts being used:

using UnityEngine;
using System;

[RequireComponent(typeof(AudioSource))]
public class Playershooting : MonoBehaviour {
	public Vector3 bulletOffset = new Vector3 (0, 0.5f, 0);

	public float attackDelay = 0.5f;
	public int attackDamage = 10;

	public GameObject playerBullet;
	GameObject enemy;
	DamageHandlerEnemy damageHandlerEnemy;
	AudioSource shotSound;
	float timer;

	void Awake () {
		enemy = GameObject.FindGameObjectWithTag ("Enemy");
		damageHandlerEnemy = enemy.GetComponent<DamageHandlerEnemy> ();
		shotSound = GetComponent<AudioSource> ();
	}

	void Update () {
		timer += Time.deltaTime;

		if (timer >= attackDelay) {
			Attack ();
		}
	}

	void Attack () {
		Vector3 offset = transform.rotation * new Vector3 (0, 0.5f, 0);

		if (Input.GetButton("Fire1")) {
		timer = 0f;
		shotSound.Play ();
		Instantiate (playerBullet, transform.position + offset, transform.rotation);
		}

		if(damageHandlerEnemy.currentEnemyHealth > 0) {
			damageHandlerEnemy.enemyDamaged (attackDamage);
		}
	}
}

using UnityEngine;
using System.Collections;

public class DamageHandlerEnemy : MonoBehaviour {

	public int startEnemyHealth = 10;
	public int currentEnemyHealth;

	bool isdamaged;
	bool isDead;

	void Awake () {
		currentEnemyHealth = startEnemyHealth;
	}

	void Update () {
		
	}

	public void enemyDamaged(int amount) {
		isdamaged = true;

		currentEnemyHealth -= amount;

		if (currentEnemyHealth <= 0 && !isDead) {
			Death ();
		}
	}

	void Death() {
		isDead = true;
		Destroy (gameObject);
	}
}

Your damageHandlerEnemy is not set. An easy way to make sure it is set, is to make it a public variable and dragging the correct object in the variable via the editor.