MissingRefrenceException help for enemyai

I have some code for a simple ai. My game has a wave system and respawning but when the player dies the game stops and I get this error MissingRefrenceException The object type “Transform” has been destroyed but you are still trying to access it. I know I have to put in a statement to check if the transform is null but I just don’t know where or what to do if it is null. Any help is appreciated, here is the code.
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class EnemyAI : MonoBehaviour
{
public Transform target;
public int moveSpeed;
public int rotationSpeed;

private Transform myTransform;

// Use this for initialization
void Awake()
{
    myTransform = transform;
}

void Start()
{
    GameObject go = GameObject.FindGameObjectWithTag("Player");

    target = go.transform;

}

// Update is called once per frame
void Update()
{
   
    Vector3 dir = target.position - myTransform.position;
    dir.z = 0.0f; // Only needed if objects don't share 'z' value
   
    if (dir != Vector3.zero)
    {
        myTransform.rotation = Quaternion.Slerp(myTransform.rotation,
                                                 Quaternion.FromToRotation(Vector3.right, dir), rotationSpeed * Time.deltaTime);
    }
    
    //Move Towards Target
    myTransform.position += (target.position - myTransform.position).normalized * moveSpeed * Time.deltaTime;
      
    
}

}

void update()
{
if (go!=null)
{
//rest of the code here
}
}