field become null after object as instantiated using EditorGUILayout.ObjectField

Hello everyone, this is my code for change text to a label in the hierarchy:

 public class Collectable : MonoBehaviour {
 public UILabel HUDLabelOwner;   //label of NGUI
 
 void Start()
 {
    HUDLabelOwner.text = "This is a label"
 }
 }
 
 [CustomEditor(typeof(Collectable))]
 public class MyScriptEditor : Editor
 {
     public override void OnInspectorGUI()
     {
         Collectable cll = (Collectable)target;
 
         cll.HUDLabelOwner = EditorGUILayout.ObjectField("HUDLabelOwner: ", cll.HUDLabelOwner, typeof(UILabel), true) as UILabel;
     }
 }

The gameobject that bring this script is a prefab, and is instantiated when I press play, while the label is already instantiated. I assign at prefab field the label by drag in from hierarchy, but when i press play all field are reset to null(prefab as well).

What can be? ?:\