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.