[cloud_firestore] how know if exist a field.

[Hakentha]

When i try use != null in a field that dont exist. i got a error.
How i must handle?

DocumentSnapshot doc
//... fill doc
print(doc['ap']); //prints ok!!

Timestamp _time = (doc['cpt'] != null) ? doc['cpt'] : Timestamp.now(); //error

Thank you!!