NullReferenceException: Object reference not set to an instance of an object. What is going on??

I want to make health system for my player and need to take damage. I follow a tutorial on how to make enemy damage the player. But suddenly this popped up:

NullReferenceException: Object reference not set to an instance of an object
EnemyAttack.Update () (at Assets/EnemyAttack.cs:64)

My player health script:

using UnityEngine;
using UnityEngine.UI;
using System.Collections;

public class PlayerHealth : MonoBehaviour
{
	public int startingHealth = 100;                            // The amount of health the player starts the game with.
	public int currentHealth;                                   // The current health the player has.
	public Slider healthSlider;                                 // Reference to the UI's health bar.
	public Image damageImage;                                   // Reference to an image to flash on the screen on being hurt.
	public AudioClip deathClip;                                 // The audio clip to play when the player dies.
	public float flashSpeed = 5f;                               // The speed the damageImage will fade at.
	public Color flashColour = new Color(1f, 0f, 0f, 0.1f);     // The colour the damageImage is set to, to flash.
	
	
	Animator anim;                                              // Reference to the Animator component.
	AudioSource playerAudio;                                    // Reference to the AudioSource component.
	PlayerMovement playerMovement;                              // Reference to the player's movement.
	//PlayerShooting playerShooting;                              // Reference to the PlayerShooting script.
	bool isDead;                                                // Whether the player is dead.
	bool damaged;                                               // True when the player gets damaged.
	
	
	void Awake ()
	{
		// Setting up the references.
		anim = GetComponent <Animator> ();
		playerAudio = GetComponent <AudioSource> ();
		playerMovement = GetComponent <PlayerMovement> ();
		//playerShooting = GetComponentInChildren <PlayerShooting> ();
		
		// Set the initial health of the player.
		currentHealth = startingHealth;
	}
	
	
	void Update ()
	{
		// If the player has just been damaged...
		if(damaged)
		{
			// ... set the colour of the damageImage to the flash colour.
			damageImage.color = flashColour;
		}
		// Otherwise...
		else
		{
			// ... transition the colour back to clear.
			damageImage.color = Color.Lerp (damageImage.color, Color.clear, flashSpeed * Time.deltaTime);
		}
		
		// Reset the damaged flag.
		damaged = false;
	}
	
	
	public void TakeDamage (int amount)
	{
		// Set the damaged flag so the screen will flash.
		damaged = true;
		
		// Reduce the current health by the damage amount.
		currentHealth -= amount;
		
		// Set the health bar's value to the current health.
		healthSlider.value = currentHealth;
		
		// Play the hurt sound effect.
		playerAudio.Play ();
		
		// If the player has lost all it's health and the death flag hasn't been set yet...
		if(currentHealth <= 0 && !isDead)
		{
			// ... it should die.
			Death ();
		}
	}
	
	
	void Death ()
	{
		// Set the death flag so this function won't be called again.
		isDead = true;
		
		// Turn off any remaining shooting effects.
		//playerShooting.DisableEffects ();
		
		// Tell the animator that the player is dead.
		anim.SetTrigger ("Die");
		
		// Set the audiosource to play the death clip and play it (this will stop the hurt sound from playing).
		playerAudio.clip = deathClip;
		playerAudio.Play ();
		
		// Turn off the movement and shooting scripts.
		playerMovement.enabled = false;
		//playerShooting.enabled = false;
	}       
}

My enemy attack script:

using UnityEngine;
using System.Collections;


public class EnemyAttack : MonoBehaviour
{
	public float timeBetweenAttacks = 0.5f;     // The time in seconds between each attack.
	public int attackDamage = 10;               // The amount of health taken away per attack.
	
	
	Animator anim;                              // Reference to the animator component.
	GameObject player;                          // Reference to the player GameObject.
	PlayerHealth playerHealth;                  // Reference to the player's health.
	//EnemyHealth enemyHealth;                    // Reference to this enemy's health.
	bool playerInRange;                         // Whether player is within the trigger collider and can be attacked.
	float timer;                                // Timer for counting up to the next attack.
	
	
	void Awake ()
	{
		// Setting up the references.
		player = GameObject.FindGameObjectWithTag ("Player");
		playerHealth = player.GetComponent <PlayerHealth> ();
		//enemyHealth = GetComponent<EnemyHealth>();
		anim = GetComponent <Animator> ();
	}
	
	
	void OnTriggerEnter (Collider other)
	{
		// If the entering collider is the player...
		if(other.gameObject == player)
		{
			// ... the player is in range.
			playerInRange = true;
		}
	}
	
	
	void OnTriggerExit (Collider other)
	{
		// If the exiting collider is the player...
		if(other.gameObject == player)
		{
			// ... the player is no longer in range.
			playerInRange = false;
		}
	}
	
	
	void Update ()
	{
		// Add the time since Update was last called to the timer.
		timer += Time.deltaTime;
		
		// If the timer exceeds the time between attacks, the player is in range and this enemy is alive...
		if(timer >= timeBetweenAttacks && playerInRange/* && enemyHealth.currentHealth > 0*/)
		{
			// ... attack.
			Attack ();
		}
		
		// If the player has zero or less health...
		if(playerHealth.currentHealth <= 0)
		{
			// ... tell the animator the player is dead.
			anim.SetTrigger ("PlayerDead");
		}
	}
	
	
	void Attack ()
	{
		// Reset the timer.
		timer = 0f;
		
		// If the player has health to lose...
		if(playerHealth.currentHealth > 0)
		{
			// ... damage the player.
			playerHealth.TakeDamage (attackDamage);
		}
	}
}

Really hope to get this answered ASAP. My deadline is almost here. Thanks !

Try placing your PlayerHealth script in a folder called Plugins. This will enable you to use GetComponent on it.

And a note for another time, please don’t post all your scripts please only post the relavant code. For example only post the function that is giving the error and the global variables that it use.