NullReferenceException: Object reference not set to an instance of an object

Using the following script I get the following error ; "NullReferenceException: Object reference not set to an instance of an object" for both lines 32 and 18 .

 using UnityEngine;
 using System.Collections;
 
 public class HealthPoints : MonoBehaviour {
 
     public int health = 1;
     public GameObject Enemy;
     ScoreManager scoreManager;
 
     // Update is called once per frame
 
     void Start(){
         scoreManager = GetComponent<ScoreManager>();
     }
 
     void Update () {
         Die ();
     }
     
     void OnTriggerEnter2D(){
 
         Debug.Log ("You hit something!");
         health--;
 
     }
 
     void Die(){
         if (health <= 0) {
             Destroy (gameObject);
             if (gameObject.layer == 8) {
                 scoreManager.score=10;
                 Debug.Log ("Enemy has died!");
             }
         }
     }
 }

Here is the "ScoreManager" Script :

 using UnityEngine;
 using System.Collections;
 
 public class ScoreManager : MonoBehaviour {
     
     public GUISkin theSkin;
     public int score;
 
     // Use this for initialization
     void Start () {
         score = 0;
     }
 
     void OnGUI(){
         GUI.skin = theSkin;
         GUI.Label (new Rect (Screen.width/2, 0, 100, 100), "" + score);
     }
 }

I'm not sure what to do as the error only appears after a trigger occurs. I believe it has something to do with the variable (int) score from the ScoreManager script, but I am unsure on how to resolve it. I have also looked at various other Unity Answers pages but despite my code being fairly similar, it still doesn't work. Thank you for reading and I appreciate any answers.