How Can I load Different Prefab on Combobox Change?

Hi! I have actually a problem to load my prefab on my scene on my dropdownlist change the error is this one. The Object you want to instantiate is null.

 Dropdown m_Dropdown;
 GameObject[] Ennemies = new GameObject[3];
 GameObject EnnemiCube;
 Button BtnRetour;
 
 void Start()
 {
     Ennemies[0] = Resources.Load("Prefabs/Prf.Ennemi1") as GameObject;
     Ennemies[1] = Resources.Load("Prefabs/Prf.Ennemi2") as GameObject;
     Ennemies[2] = Resources.Load("Prefabs/Prf.Ennemi3") as GameObject;

     EnnemiCube = GameObject.Find("TemplateEnnemi");
     EnnemiCube = Ennemies[0];

     Instantiate(EnnemiCube, new Vector3(- 28f, 10f, 85f), Quaternion.identity);
     EnnemiCube.transform.localScale = new Vector3(20f, 20f, 20f);

     m_Dropdown = GetComponentInChildren<Dropdown>();
     m_Dropdown.onValueChanged.AddListener(delegate { DropdownValueChanged(m_Dropdown); });

     BtnRetour = GameObject.Find("BtnRetour").GetComponent<Button>();
     BtnRetour.onClick.AddListener(() => OuvrirSceneMenu());
 }
   
 void Update()
 {
     Rotation();
 }
 public void OuvrirSceneMenu()
 {
     SceneManager.LoadScene("Scn.Menu", LoadSceneMode.Single);
 }
 public void Rotation() => Ennemies[m_Dropdown.value].transform.Rotate(new Vector3(0, Input.GetAxis("Horizontal"), 0));

 public void DropdownValueChanged(Dropdown dropdown)
 {
     switch (dropdown.value)
     {
         case 0 :
             EnnemiCube = Instantiate(Ennemies[0], transform.position, Quaternion.identity) as GameObject; 
             break;
         case 1:
             EnnemiCube = Instantiate(Ennemies[1], transform.position, Quaternion.identity) as GameObject;
             break;
         case 2:
             EnnemiCube = Instantiate(Ennemies[2], transform.position, Quaternion.identity) as GameObject;
             break;
     }
     
 }

Thanks!