NullReferenceException

"NullReferenceException: Object reference not set to an instance of an object ObjectDetect2.OnDisable () "

How do I resolve the error?

 function OnEnable() {
     var gameController = GameObject.FindWithTag("EditorOnly");
     gameController.SendMessage("OnEnableObjective");
 }
  
 function OnDisable() {
     var gameController = GameObject.FindWithTag("EditorOnly");
     gameController.SendMessage("OnDisableObjective");
 }
 

other script

 function OnEnableObjective() {
     objectives++;
    
 }
  
 function OnDisableObjective() {
     objectives--;
     if (objectives == 0)
         SendMessage("OnObjectives");
 }