Update field in all documents by looping in Cloud Firestore collection in Flutter

Question

I'm trying to loop through all documents in a collection and update one field in all of them.

Getting the field, and adding 1 hour:

DateTime parsedDateTime(DocumentSnapshot documentSnapshot) {
  Timestamp timestamp = documentSnapshot.get('Start Time');
  late DateTime d = timestamp.toDate().add(Duration(hours: 1));
  return d;
}

My (attempt at a) loop:

Future<dynamic> pushedPulledTime(DocumentSnapshot<Object?> snapshot) async {
      FirebaseFirestore.instance
          .collection('products')
          .get()
          .then((querySnapshot) {
        for (var doc in querySnapshot.docs) {
          doc.reference.update({
            'Start Time': parsedDateTime(snapshot),
          });
        }
      });
    }

And my button:

return Container(child: FutureBuilder<DocumentSnapshot>(
        builder: (BuildContext context, snapshot) {
      return ElevatedButton(              
          onPressed: () async {
            await pushedPulledTime(snapshot.data!);

The error I get on pressed:

E/flutter (25251): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Null check operator used on a null value

I should add that I placed one document in the 'products' collection, with a timestamp in the 'Start Time' field, so I know it's not getting an empty value from firestore.