why unity object should not use null propagation

                
                        Unity has overridden the == operator for UnityEngine.Object. 
If you use the == operator to compare a UnityEngine.Object to null,
it will return true if the UnityEngine.Object is destroyed, even if
  the object itself isn't actually null. Null propagation cannot be
  overridden in this way, and therefore behaves inconsistently with
  the == operator, because it checks for null in a different way.