Issues instantiating a game object

Hello, I am instantiating a list of UI game objects. But when I start the game I get a

null reference exception: object reference not set to an instance of an object

Here is my code:

     public class MyClass: Monobehaviour
     {
         public GameObject ObjectiveTextPrefab;
         private GameObject _objectiveListInstance;
         
             void Start()
             {
                 _data = GameData.Instance.LoadData();
                
                 var stats = _data.Scenarios.FirstOrDefault(t => t.Name == Scenario.ToString());
         
                 for (int i = 0; i < stats.MyObjectives.Length; i++)
                 {
                     _objectiveListInstance = Instantiate(ObjectiveTextPrefab);
                     _objectiveListInstance.transform.SetParent(objectivesListParent, false);
                     _objectiveListInstance.transform.Find("ObjectiveText").GetComponent<Text>().text = stats.MyObjectives[i].Title;
                     _objectiveListInstance.transform.Find("Counter").GetComponent<Text>().text = stats.MyObjectives[i].Count.ToString();
                     
                 }
             }
 void OnEnable()
     {
         _data = GameData.Instance.LoadData();
 
         var stats = _data.Scenarios.FirstOrDefault(t => t.Name == Scenario.ToString());
 
         for (int i = 0; i < stats.MyObjectives.Length; i++)
         {
             if (stats.MyObjectives[i].Active)
             {
                 _objectiveListInstance.transform.Find("Counter").GetComponent<Text>().color = Color.yellow;
                 _objectiveListInstance.transform.Find("ObjectiveText").GetComponent<Text>().color = Color.yellow;
             }
             else
             {
                 _objectiveListInstance.transform.Find("ObjectiveText").GetComponent<Text>().color = Color.white;
                 _objectiveListInstance.transform.Find("Counter").GetComponent<Text>().color = Color.white;
             }
 
             if (stats.MyObjectives[i].Active)
             {
                 _objectiveListInstance.transform.Find("Counter").GetComponent<Text>().color = Color.yellow;
                 _objectiveListInstance.transform.Find("ObjectiveText").GetComponent<Text>().color = Color.yellow;
             }
             else
             {
                 _objectiveListInstance.transform.Find("ObjectiveText").GetComponent<Text>().color = Color.white;
                 _objectiveListInstance.transform.Find("Counter").GetComponent<Text>().color = Color.white;
             }
             
             if (stats.MyObjectives[i].Completed)
             {
                 _objectiveListInstance.transform.Find("Toggle").GetComponent<Toggle>().isOn = true;
                 _objectiveListInstance.transform.Find("ObjectiveText").GetComponent<Text>().color = Color.gray;
                 _objectiveListInstance.transform.Find("Counter").GetComponent<Text>().color = Color.gray;
             }
             else
             {
                 _objectiveListInstance.transform.Find("Toggle").GetComponent<Toggle>().isOn = false;
             }
         }
 }
 
     }

The error points to the _objectiveListInstance variable found in the OnEnable() function. It says that it is null. Can someone help me understand why I am getting this issue? Many thanks in advance!