By default variables marked with SerializeReference will not show unless they have already been assigned.
Unity provides no drawer for null, and does not show an object picker in this circumstance.


Assign a variable manually#

Variables initialised via code or via editor scripts should appear correctly if they are properly serializable.

Assign the variable via Editor UI#

Either write your own editor code, or use a custom property drawer like Vertx.SerializeReferenceDropdown to pick an instance of a type from the editor.