Flutter : firebase MissingPluginException - flutter

Question

I'm having this problem about flutter and firebase(realtime-database).
Unhandled Exception: MissingPluginException(No implementation found for method Query#observe on channel plugins.flutter.io/firebase_database)
Here is the error in terminal
error
// error message
[VERBOSE-2:ui_dart_state.cc(157)] Unhandled Exception: MissingPluginException(No implementation found for method Query#observe on channel plugins.flutter.io/firebase_database)
#0 MethodChannel._invokeMethod (package:flutter/src/services/platform_channel.dart:154:7)
<asynchronous suspension>
#1 MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:329:12)
#2 Query._observe.<anonymous closure> (package:firebase_database/src/query.dart:50:38)
#3 _runGuarded (dart:async/stream_controller.dart:820:24)
#4 _BroadcastStreamController._subscribe (dart:async/broadcast_stream_controller.dart:215:7)
#5 _ControllerStream._createSubscription (dart:async/stream_controller.dart:833:19)
#6 _StreamImpl.listen (dart:async/stream_impl.dart:475:9)
#7 Stream.first (dart:async/stream.dart:1254:25)
#8 Query.once (package:firebase_database/src/query.dart:84:55)
#9 _IoTState.initState (package:flutter_try_iot/main.dart:36:13)
#10 StatefulElement._f<…>
I'm trying to get data from firebase to display in flutter app.
Here is my code main.dart
class _IoTState extends State<IoT> {
final getIot = FirebaseDatabase.instance.reference();
final getData = FirebaseDatabase.instance.reference().child('xxx-xxx');
String temperatue = "0";
String illuminance = "0";
List<String> temp = [];
List<String> light = [];
#override
void initState(){
getData.once().then((DataSnapshot snapshot){
if (snapshot.value == null) {
print("Item doesn't exist in the db");
} else {
print("Item exists in the db");
}
});
}
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
backgroundColor: Color(0xFF7ACADA),
title: Text(
"IoT ZigBee Simulation",
textAlign: TextAlign.center,
style: TextStyle(
color: Color(0xFF1A1244)
),
),
),
body: Column(
),
);
}
}
pubspec.yaml
firebase_core: ^0.4.5
firebase_database: ^3.1.6
flutter_icons: ^1.1.0
fl_chart: ^0.10.1
I follow instructions to add ios to firebase .ios add firebase
no issues found by flutter doctor
I follow other peoples suggestion to do flutter clean & flutter packages get, but still not able to fix this problem
https://github.com/flutter/flutter/issues/13971
terminal
Also pop up a window telling me The file “Runner.xcworkspace” has been modified by another application" everytime I execute flutter clean/ flutter packages get
I successfully connect to firebase yesterday, but after I close my simulator I start to receive error message
success picture
Please give me some solution or advice to fix this problem, thanks.
Please tell me if I need to provide additional information