type '(DataSnapshot) => Null' is not a subtype of type '(DatabaseEvent) => dynamic' in type cast

[Rcharis]

class Dashboard extends StatefulWidget {
@OverRide
_DashboardState createState() => _DashboardState();
}

class _DashboardState extends State with SingleTickerProviderStateMixin {

bool isLoading = false;
final GoogleSignIn googleSignIn = GoogleSignIn();

final databaseReference = FirebaseDatabase.instance.reference();

late AnimationController progressController;
late Animation tempAnimation;
late Animation humidityAnimation;

@OverRide
void initState() {
super.initState();

DatabaseReference databaseReference = FirebaseDatabase.instance.reference();
databaseReference.child('sensor').once().then((DataSnapshot snapshot) {
  Map<String, dynamic>? data = snapshot.value as Map<String, dynamic>?;

  if (data != null) {
    double temp = data['suhu']['Sensor_T'].toDouble();
    double humidity = data['kelembaban']['Sensor_H'].toDouble();

    setState(() {
      isLoading = true;
    });

    _DashboardInit(temp, humidity);
  }
} as FutureOr Function(DatabaseEvent readEvent));

}