The method was called on null - firebase

Question

I am trying to use firebase analytics to follow the user navigation and pressing on some buttons i made a _currentScreen() function which i call init state in the class in which the user navigates to
here is the function
Future<Null> _currentScreen() async {
await widget.analytics.setCurrentScreen(
screenName: 'second screen view', screenClassOverride: 'SecondScreenView');
}
and i made another function which is _sendAnalytics which i call when the user click on a button; here is the function:
Future<Null> _sendAnalytics() async {
await widget.analytics
.logEvent(name: 'launchhhh', parameters: <String, dynamic>{});
}
I have several problems:
1st- in the initstate in the second screen class it's warning :is method overrides a method annotated as #mustCallSuper in 'State', but does not invoke the overridden method.
2nd- when i run the code and press the button of navigations i have this
[ERROR:flutter/shell/common/shell.cc(181)] Dart Error: Unhandled exception:
E/flutter (12744): NoSuchMethodError: The method 'logEvent' was called on null.
E/flutter (12744): Receiver: null
E/flutter (12744): Tried calling: logEvent(name: "launchhhh", parameters: _LinkedHashMap len:0)
E/flutter (12744): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
E/flutter (12744): #1 FirstScreenState._sendAnalytics (file:///C:/Users/pc/Desktop/wedn2/wedn2/lib/h.dart:40:10)
3- Also, when i comment the _sendanalytics function
NoSuchMethodError: The method 'setCurrentScreen' was called on null.
E/flutter (15612): Receiver: null
E/flutter (15612): Tried calling: setCurrentScreen(screenClassOverride: "SecondScreenView", screenName: "second screen view")
E/flutter (15612): #0 Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:50:5)
E/flutter (15612): #1 SecondScreenState._currentScreen (file:///C:/Users/pc/Desktop/wedn2/wedn2/lib/h.dart:91:28)
E/flutter (15612): <asynchronous suspension>
E/flutter (15612): #2 SecondScreenState.initState (file:///C:/Users/pc/Desktop/wedn2/wedn2/lib/h.dart:88:5)
E/flutter (15612): #3 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3787:58)
E/flutter (15612): #4 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3653:5)
I don't know where is the problem any help? and here is my whole code:
import 'package:flutter/material.dart';
import 'package:firebase_analytics/firebase_analytics.dart';
import 'package:firebase_analytics/observer.dart';
import 'dart:async';
class MyAppf extends StatelessWidget {
static FirebaseAnalytics analytics = new FirebaseAnalytics();
static FirebaseAnalyticsObserver observer =
new FirebaseAnalyticsObserver(analytics: analytics);
#override
Widget build(BuildContext context) {
return new MaterialApp(
debugShowCheckedModeBanner: false,
title: 'Flutter Demo',
theme: new ThemeData(
primarySwatch: Colors.blue,
),
navigatorObservers: <NavigatorObserver>[observer],
// home: new WallScreen(analytics: analytics, observer: observer),
home: new FirstScreen(),
);
}
}
class FirstScreen extends StatefulWidget {
final FirebaseAnalytics analytics;
final FirebaseAnalyticsObserver observer;
FirstScreen({this.analytics, this.observer});
#override
FirstScreenState createState() => new FirstScreenState();
}
class FirstScreenState extends State<FirstScreen> {
Future<Null> _sendAnalytics() async {
await widget.analytics
.logEvent(name: 'launchhhh', parameters: <String, dynamic>{});
}
final FirebaseAnalytics analytics;
final FirebaseAnalyticsObserver observer;
FirstScreenState({this.analytics, this.observer});
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text('First Screen'),
),
body: Center(
child: RaisedButton(
child: Text('Launch screen'),
onPressed: () {
Navigator.push(
context,
MaterialPageRoute(builder: (context) => SecondScreen()),
);
_sendAnalytics();
},
),
),
);
}
}
class SecondScreen extends StatefulWidget {
final FirebaseAnalytics analytics;
final FirebaseAnalyticsObserver observer;
SecondScreen({this.analytics, this.observer});
#override
SecondScreenState createState() => new SecondScreenState();
}
class SecondScreenState extends State<SecondScreen> {
#override
void initState() {
_currentScreen();
}
Future<Null> _currentScreen() async {
await widget.analytics.setCurrentScreen(
screenName: 'second screen view', screenClassOverride: 'SecondScreenView');
}
Future<Null> _sendAnalyticsback() async {
await widget.analytics
.logEvent(name: 'back', parameters: <String, dynamic>{});
}
final FirebaseAnalytics analytics;
final FirebaseAnalyticsObserver observer;
SecondScreenState({this.analytics, this.observer});
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Second Screen"),
),
body: Center(
child: RaisedButton(
onPressed: () {
Navigator.pop(context);
_sendAnalyticsback();
},
child: Text('Go back!'),
),
),
);
}
}
EDIT I passed my var analytics to Widgets to be
home:new FirstScreen(analytics: analytics),
Navigator.push(
context,
MaterialPageRoute(builder: (context) => SecondScreen(analytics:analytics)),
instead of :
home:new FirstScreen(),
Navigator.push(
context,
MaterialPageRoute(builder: (context) => SecondScreen()),
But still it's the same error calling on null

Answer 1

You need to pass your var analytics to your Widgets.
new FirstScreen(),
new SecondScreen(),
Should be :
new FirstScreen(analytics: analytics),
new SecondScreen(analytics: analytics),

Answer 2

Do not use static members as they are intended to be accessed through the class itself. A cealner way of what you are trying to do is declare MyApp as a SatetfulWidget , now you analytics and observer can be private members of State class and use initState to initialize them, see the following example:
class MyApp extends StatefulWidget {
#override
_MyAppState createState() => new _MyAppState();
}
class _MyAppState extends State<MyApp> {
FirebaseAnalytics _analytics;
FirebaseAnalyticsObserver _observer ;
#override
void initState() {
_analytics = FirebaseAnalytics();
_observer = FirebaseAnalyticsObserver(analytics: _analytics);
super.initState();
}
#override
Widget build(BuildContext context) {
return Container();
}
}
If you want to keep your widget Stateless you can initialize your observer in the constructor's initializer list, this requires the analytics field to be passed a value when MyApp is created MyApp(analytics:FirebaseAnalytics()):
class MyApp extends StatelessWidget {
final FirebaseAnalytics analytics;
final FirebaseAnalyticsObserver observer;
MyApp({Key key,#required this.analytics})
: observer = FirebaseAnalyticsObserver(analytics: analytics),
super(key: key);
#override
Widget build(BuildContext context) {
return Container();
}
}