Enemy tracking Script Help

I DO NOT OWN THIS SCRIPT***

It says "MissingRefenceException: the object type 'Transform' has been destroyed but you are still trying to access it. Your script should either check if it is null or you should not destroy the object.

 using UnityEngine;
 using System.Collections;
 
 public class Enemy_AI : MonoBehaviour {
 
     Transform tr_Player;
     float f_RotSpeed=3.0f,f_MoveSpeed = 5.0f;
     
     // Use this for initialization
     void Start () {
         
         tr_Player = GameObject.FindGameObjectWithTag ("Player").transform; }
     
     // Update is called once per frame
     void Update () {
         /* Look at Player*/
         transform.rotation = Quaternion.Slerp (transform.rotation , Quaternion.LookRotation (tr_Player.position - transform.position) , f_RotSpeed * Time.deltaTime);
         
         /* Move at Player*/
         transform.position += transform.forward * f_MoveSpeed * Time.deltaTime;
     }
 }

How do I fix this?