Null Reference

These scripts were working earlier but now it says that there is a null refference.

NullReferenceException: Object reference not set to an instance of an object PlayerHealth.AddHealth (Int32 amount) (at Assets/Scripts/Player/PlayerHealth.cs:56) PlayerAddHealth.OnTriggerEnter (UnityEngine.Collider player) (at Assets/Scripts/Player/PlayerAddHealth.cs:24)

 using UnityEngine;
 using System.Collections;
 
 public class PlayerAddHealth : MonoBehaviour
 {
     public int addHealth = 10;
     public int startingHealth = 100;
     public int currentHealth;
 
     public GameObject player;
     public PlayerHealth playerHealth;
     
     void Awake ()
     {
         player = GameObject.FindGameObjectWithTag ("Player");
         playerHealth = player.GetComponent<PlayerHealth> ();
     }
 
     void OnTriggerEnter (Collider player)
     {
         if (playerHealth.currentHealth > 0)
         {
             playerHealth.AddHealth (addHealth);
         }
     }
 }
 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 

I dont really see the problem in any of these but I'm wrong so help would be appreciated.

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class PlayerHealth : MonoBehaviour
 {
     public int startingHealth = 100;
     public int currentHealth;
     public int maxHealth = 100;
     public Slider HealthSlider;
     public Image damageImage;
     public AudioClip deathClip;
     public float flashSpeed = 5f;
     public Color flashColour = new Color(1f, 0f, 0f, 0.1f);
 
     Animator anim;
     AudioSource playerAudio;
     PlayerMovement playerMovement;
     PlayerShooting playerShooting;
     bool isDead;
     bool damaged;
 
 
     void Awake ()
     {
         anim = GetComponent <Animator> ();
         playerAudio = GetComponent <AudioSource> ();
         playerMovement = GetComponent <PlayerMovement> ();
         playerShooting = GetComponentInChildren <PlayerShooting> ();
         currentHealth = startingHealth;
     }
 
     void Start ()
     {
         currentHealth = maxHealth;
     }
 
      void Update ()
     {
         if(damaged)
         {
             damageImage.color = flashColour;
         }
         
         else
         {
             damageImage.color = Color.Lerp (damageImage.color, Color.clear, flashSpeed * Time.deltaTime);
         }
         damaged = false;
     }
 
     public void AddHealth (int amount)
     {
         currentHealth += amount;
 
         HealthSlider.value = currentHealth;
 
         if (currentHealth <= 0) 
         {
             currentHealth = 0;
         }
         
         if (currentHealth >= maxHealth)
         {
             currentHealth = maxHealth;
         }
     }
 
     public void TakeDamage (int amount)
     {
         damaged = true;
 
         currentHealth -= amount;
 
         HealthSlider.value = currentHealth;
 
         playerAudio.Play ();
         
         if (currentHealth <= 0 && !isDead)
         {
             Death ();
         }
     }
     
     public void Death ()
     {
         isDead = true;
 
         playerShooting.DisableEffects ();
 
         anim.SetTrigger ("Die");
 
         playerAudio.clip = deathClip;
         playerAudio.Play ();
 
         playerMovement.enabled = false;
         playerShooting.enabled = false;
     }
 
     public void RestartLevel ()
     {
         Application.LoadLevel (Application.loadedLevel);
     }
 }