Hide gameobject until certain condition has been met,Hide an object until a condition is met.

I think you should create a GameManager, always enabled, responsible for detecting when your enemies are destroyed. Then, your Door script will be warned by the GameManager once all the enemies are destroyed.

 // Enemy.cs
 using UnityEngine;
 
 public class Enemy : MonoBehaviour, IPointerClickHandler
 {
     public event System.Action<Enemy> OnKilled;
 
     void OnDisable()
     {
         if ( OnKilled != null )
             OnKilled.Invoke( this );
     }
 }


 // GameManager.cs
 using UnityEngine;
 
 public class GameManager : MonoBehaviour
 {
     private int remainingEnemies = 0 ;
     public event System.Action OnLastEnemyKilled;
 
     private void Start()
     {
         Enemy[] enemies = FindObjectsOfType<Enemy>();
         for ( int i = 0 ; i < enemies.Length ; i++ )
         {
             AddEnemy( enemies[i] ) ;
         }
     }
 
     private void AddEnemy( Enemy enemy )
     {
         remainingEnemies++ ;
         enemy.OnKilled += RemoveEnemy ;
     }
 
     private void RemoveEnemy( Enemy enemy )
     {
         enemy.OnKilled -= RemoveEnemy ;
         remainingEnemies-- ;
         if( remainingEnemies == 0 && OnLastEnemyKilled != null )
             OnLastEnemyKilled.Invoke() ;
     }
 }

 // Door.cs
 using UnityEngine;
 
 public class Door: MonoBehaviour
 {
     // Drag & drop the object in the inspector
     public GameManager GameManager ;
 
     private void Awake()
     {
         GameManager.OnLastEnemyKilled += Show ;
         Hide();
     }
 
     private void Show()
     {
         GameManager.OnLastEnemyKilled -= Show ;
         gameObject.SetActive( true ) ;
     }
 
     private void Hide()
     {
         gameObject.SetActive( false ) ;
     }
 }

ORIGINAL ANSWER

Simply disable the gameobject instead of disabling the Renderer component:

  gameObject.SetActive( GameObject.FindWithTag("Enemy") == null ) ;

However, keep in mind that calling the FindXXX functions are not very efficient. If you have only few calls in your scene, it's fine.

If you don't want to disable the gameObject as @Hellium suggests, you need to enable & disable the MeshRenderer component.

 gameObject.GetComponent<MeshRenderer>().enabled = true;

By the way: the GetComponent function is called every frame in the Update function. It's terrible for performance. You should store the MeshRenderer in a variable.