final

lib/utils/double_back_to_close_app.dart

@@ -21,9 +21,9 @@ class DoubleBackToCloseApp extends StatefulWidget {
   /// Creates a widget that allows the user to close the app by double tapping
   /// the back-button.
   const DoubleBackToCloseApp({
-    Key key,
-    @required this.snackBar,
-    @required this.child,
+    Key? key,
+    required this.snackBar,
+    required this.child,
   })  : assert(snackBar != null),
         assert(child != null),
         super(key: key);
@@ -34,7 +34,7 @@ class DoubleBackToCloseApp extends StatefulWidget {
 
 class _DoubleBackToCloseAppState extends State<DoubleBackToCloseApp> {
   /// The last time the user tapped Android's back-button.
-  DateTime _lastTimeBackButtonWasTapped;
+  DateTime? _lastTimeBackButtonWasTapped;
 
   /// Returns whether the current platform is Android.
   bool get _isAndroid => Theme.of(context).platform == TargetPlatform.android;
@@ -50,7 +50,7 @@ class _DoubleBackToCloseAppState extends State<DoubleBackToCloseApp> {
   bool get _isSnackBarVisible =>
       (_lastTimeBackButtonWasTapped != null) &&
           (widget.snackBar.duration >
-              DateTime.now().difference(_lastTimeBackButtonWasTapped));
+              DateTime.now().difference(_lastTimeBackButtonWasTapped!));
 
   /// Returns whether the next back navigation of this route will be handled
   /// internally.
@@ -59,7 +59,7 @@ class _DoubleBackToCloseAppState extends State<DoubleBackToCloseApp> {
   /// local-history of the current route, in order to handle pop. This is done
   /// by [Drawer], for example, so it can close on pop.
   bool get _willHandlePopInternally =>
-      ModalRoute.of(context).willHandlePopInternally;
+      ModalRoute.of(context)!.willHandlePopInternally;
 
   @override
   Widget build(BuildContext context) {