Make ReadMe examples null-safe & fix errors.

README.md

@@ -434,40 +434,41 @@ Unable to find a matching variant of project :unityLibrary:
 
 ```dart
 import 'package:flutter/material.dart';
-import 'package:flutter/services.dart';
 import 'package:flutter_unity_widget/flutter_unity_widget.dart';
 
 void main() {
-  runApp(MaterialApp(
-    home: UnityDemoScreen()
-  ));
+  runApp(
+    const MaterialApp(
+      home: UnityDemoScreen(),
+    ),
+  );
 }
 
 class UnityDemoScreen extends StatefulWidget {
-
-  UnityDemoScreen({Key key}) : super(key: key);
+  const UnityDemoScreen({Key? key}) : super(key: key);
 
   @override
-  _UnityDemoScreenState createState() => _UnityDemoScreenState();
+  State<UnityDemoScreen> createState() => _UnityDemoScreenState();
 }
 
-class _UnityDemoScreenState extends State<UnityDemoScreen>{
+class _UnityDemoScreenState extends State<UnityDemoScreen> {
   static final GlobalKey<ScaffoldState> _scaffoldKey =
       GlobalKey<ScaffoldState>();
-  UnityWidgetController _unityWidgetController;
+  UnityWidgetController? _unityWidgetController;
 
+  @override
   Widget build(BuildContext context) {
-
     return Scaffold(
       key: _scaffoldKey,
       body: SafeArea(
         bottom: false,
         child: WillPopScope(
-          onWillPop: () {
+          onWillPop: () async {
             // Pop the category page if Android back button is pressed.
+            return true;
           },
           child: Container(
-            color: colorYellow,
+            color: Colors.yellow,
             child: UnityWidget(
               onUnityCreated: onUnityCreated,
             ),
@@ -479,9 +480,10 @@ class _UnityDemoScreenState extends State<UnityDemoScreen>{
 
   // Callback that connects the created controller to the unity controller
   void onUnityCreated(controller) {
-    this._unityWidgetController = controller;
+    _unityWidgetController = controller;
   }
 }
+
 ```
 <br />
 
@@ -491,17 +493,19 @@ class _UnityDemoScreenState extends State<UnityDemoScreen>{
 import 'package:flutter/material.dart';
 import 'package:flutter_unity_widget/flutter_unity_widget.dart';
 
-void main() => runApp(MyApp());
+void main() => runApp(const MyApp());
 
 class MyApp extends StatefulWidget {
+  const MyApp({Key? key}) : super(key: key);
+
   @override
-  _MyAppState createState() => _MyAppState();
+  State<MyApp> createState() => _MyAppState();
 }
 
 class _MyAppState extends State<MyApp> {
   static final GlobalKey<ScaffoldState> _scaffoldKey =
       GlobalKey<ScaffoldState>();
-  UnityWidgetController _unityWidgetController;
+  UnityWidgetController? _unityWidgetController;
   double _sliderValue = 0.0;
 
   @override
@@ -526,10 +530,10 @@ class _MyAppState extends State<MyApp> {
           child: Stack(
             children: <Widget>[
               UnityWidget(
-                  onUnityCreated: onUnityCreated,
-                  onUnityMessage: onUnityMessage,
-                  onUnitySceneLoaded: onUnitySceneLoaded,
-                  fullscreen: false,
+                onUnityCreated: onUnityCreated,
+                onUnityMessage: onUnityMessage,
+                onUnitySceneLoaded: onUnitySceneLoaded,
+                fullscreen: false,
               ),
               Positioned(
                 bottom: 20,
@@ -539,8 +543,8 @@ class _MyAppState extends State<MyApp> {
                   elevation: 10,
                   child: Column(
                     children: <Widget>[
-                      Padding(
-                        padding: const EdgeInsets.only(top: 20),
+                      const Padding(
+                        padding: EdgeInsets.only(top: 20),
                         child: Text("Rotation speed:"),
                       ),
                       Slider(
@@ -567,7 +571,7 @@ class _MyAppState extends State<MyApp> {
 
   // Communcation from Flutter to Unity
   void setRotationSpeed(String speed) {
-    _unityWidgetController.postMessage(
+    _unityWidgetController?.postMessage(
       'Cube',
       'SetRotationSpeed',
       speed,
@@ -581,15 +585,17 @@ class _MyAppState extends State<MyApp> {
 
   // Callback that connects the created controller to the unity controller
   void onUnityCreated(controller) {
-    this._unityWidgetController = controller;
+    _unityWidgetController = controller;
   }
 
   // Communication from Unity when new scene is loaded to Flutter
-  void onUnitySceneLoaded(SceneLoaded sceneInfo) {
-    print('Received scene loaded from unity: ${sceneInfo.name}');
-    print('Received scene loaded from unity buildIndex: ${sceneInfo.buildIndex}');
+  void onUnitySceneLoaded(SceneLoaded? sceneInfo) {
+    if (sceneInfo != null) {
+      print('Received scene loaded from unity: ${sceneInfo.name}');
+      print(
+          'Received scene loaded from unity buildIndex: ${sceneInfo.buildIndex}');
+    }
   }
-
 }
 
 ```