The method '[]' can't be unconditionally invoked because the receiver can be 'null'. problems with safety null – Flutter

enter image description here this the controller class :
i had the same problems with getvehicle so i used as documentSnapshot to resolve it

class TripsController {
  FirebaseFirestore _firestore = FirebaseFirestore.instance;

  Stream<List<Trips>> getTrips() {
    return _firestore
        .collection('trips')
        .snapshots()
        .map((snapshot) => snapshot.docs
        .map((doc) => Trips(
      id: doc.id,
      departure: doc.data()?['departure'],
      destination: doc.data()?['destination'],
      vehicleID: doc.data()?['vehicleID'],
    ))
        .toList());
  }

  Future<Vehicle> getVehicle(String id) async {
    DocumentSnapshot snapshot = await _firestore.collection('vehicle').doc(id).get();
    return Vehicle(
      id: snapshot.id,
      Driver: (snapshot.data as DocumentSnapshot)?['Driver'],
      name: (snapshot.data as DocumentSnapshot)?['name'],
      type: (snapshot.data as DocumentSnapshot)?['type'],
    );
  }
}
and this is the model 
class Trips {
  final String id ;
  final String departure;
  final String destination;
  final String vehicleID;

  Trips({required this.id,required this.departure, required this.destination, required this.vehicleID});
}

class Vehicle {
  final String id ;
  final String Driver;
  final String name;
  final String type;

  Vehicle({required this.id ,required this.Driver, required this.name, required this.type});
}

am having a problems with null safety everywhere