Getting NullReferenceException when setting the rotation of my enemy

I'll be short and concise. Here's the script:

 using UnityEngine;
 
 public class EnemyAI : MonoBehaviour {
     Transform target;
     public float moveSpeed = 3;
     public int rotationSpeed = 3;
 
     private bool checkPlayerDistance = true;
     private float[] playerDistance = new float[4];
     private bool isReady;
 
     Transform[] player = new Transform[4];
     
     Transform myTransform;
     
     bool losingHealth;
     
     void Awake()
     {
         myTransform = transform;
     }
     
     void Start()
     {
         target = player [1];
     }
     
     void Update () {
 
         if (checkPlayerDistance) {
             for(int i = 0; i < 4; i++){
                 if(!player[i] == null){
                     playerDistance[i] = Vector3.Distance(player[i].position, myTransform.position);
                 }
             }
         }
 
         if (checkPlayerDistance) {
             for(int i = 0; i < 4; i++){
                 if(Mathf.Min(playerDistance[0],playerDistance[1],playerDistance[2],playerDistance[3]) == playerDistance[i]){
                     target = player[i];
 
                     if(!isReady)
                         isReady = true;
                 }
             }
         }
         if (isReady) {
                         myTransform.rotation = Quaternion.Slerp (myTransform.rotation, Quaternion.LookRotation (target.position - myTransform.position), rotationSpeed * Time.deltaTime);
                         myTransform.position += myTransform.forward * moveSpeed * Time.deltaTime;
                 }
     }
 
     void OnCollisionEnter(Collision collision){
     }
 }

Where I get the error is on the part where if(isReady) is, when setting myTransform.rotation. Any guess on what could cause this?