diff --git a/chapter_06/lib/controllers/plan_controller.dart b/chapter_06/lib/controllers/plan_controller.dart
index acdb45f..e85f472 100644
--- a/chapter_06/lib/controllers/plan_controller.dart
+++ b/chapter_06/lib/controllers/plan_controller.dart
@@ -23,7 +23,7 @@ class PlanController {
     services.delete(plan);
   }
 
-  void createNewTask(Plan plan, [String description]) {
+  void createNewTask(Plan plan, [String? description]) {
     if (description == null || description.isEmpty) {
       description = 'New Task';
     }
diff --git a/chapter_06/lib/main.dart b/chapter_06/lib/main.dart
index ce4d50f..b78963a 100644
--- a/chapter_06/lib/main.dart
+++ b/chapter_06/lib/main.dart
@@ -2,14 +2,16 @@ import 'package:flutter/material.dart';
 import 'plan_provider.dart';
 import 'views/plan_creator_screen.dart';
 
-void main() => runApp(PlanProvider(child: MasterPlanApp()));
+void main() => runApp(PlanProvider(child: const MasterPlanApp()));
 
 class MasterPlanApp extends StatelessWidget {
+  const MasterPlanApp({Key? key}) : super(key: key);
+
   @override
   Widget build(BuildContext context) {
     return MaterialApp(
-        theme: ThemeData(primarySwatch: Colors.purple),
-        home: PlanCreatorScreen(),
+      theme: ThemeData(primarySwatch: Colors.purple),
+      home: const PlanCreatorScreen(),
     );
   }
-}
\ No newline at end of file
+}
diff --git a/chapter_06/lib/models/data_layer.dart b/chapter_06/lib/models/data_layer.dart
index 99b5f7f..c706bb9 100644
--- a/chapter_06/lib/models/data_layer.dart
+++ b/chapter_06/lib/models/data_layer.dart
@@ -1,2 +1,2 @@
 export 'plan.dart';
-export 'task.dart';
\ No newline at end of file
+export 'task.dart';
diff --git a/chapter_06/lib/models/plan.dart b/chapter_06/lib/models/plan.dart
index bb8f91d..8a148ed 100644
--- a/chapter_06/lib/models/plan.dart
+++ b/chapter_06/lib/models/plan.dart
@@ -1,18 +1,17 @@
 import '../repositories/repository.dart';
-import 'package:flutter/foundation.dart';
 import './task.dart';
 
 class Plan {
   final int id;
-  String name = '';
+  String name;
   List<Task> tasks = [];
 
-  Plan({@required this.id, this.name = ''});
+  Plan({required this.id, this.name = ''});
 
   Plan.fromModel(Model model)
       : id = model.id,
-        name = model?.data['name'],
-        tasks = model?.data['task']
+        name = model.data['name'] ?? '',
+        tasks = model.data['tasks']
                 ?.map<Task>((task) => Task.fromModel(task))
                 ?.toList() ??
             <Task>[];
diff --git a/chapter_06/lib/models/task.dart b/chapter_06/lib/models/task.dart
index 6a0db87..d039e11 100644
--- a/chapter_06/lib/models/task.dart
+++ b/chapter_06/lib/models/task.dart
@@ -1,5 +1,4 @@
 import '../repositories/repository.dart';
-import 'package:flutter/foundation.dart';
 
 class Task {
   final int id;
@@ -7,7 +6,7 @@ class Task {
   bool complete;
 
   Task({
-    @required this.id,
+    required this.id,
     this.complete = false,
     this.description = '',
   });
diff --git a/chapter_06/lib/plan_provider.dart b/chapter_06/lib/plan_provider.dart
index 41d307b..83cade1 100644
--- a/chapter_06/lib/plan_provider.dart
+++ b/chapter_06/lib/plan_provider.dart
@@ -4,13 +4,15 @@ import 'controllers/plan_controller.dart';
 class PlanProvider extends InheritedWidget {
   final _controller = PlanController();
 
-  PlanProvider({Key key, Widget child}) : super(key: key, child: child);
-
   @override
-  bool updateShouldNotify(InheritedWidget oldWidget) => false;
+  bool updateShouldNotify(covariant InheritedWidget oldWidget) => false;
+
+  PlanProvider({Key? key, required Widget child})
+      : super(key: key, child: child);
 
   static PlanController of(BuildContext context) {
-    PlanProvider provider = context.dependOnInheritedWidgetOfExactType<PlanProvider>();
+    final provider = context.dependOnInheritedWidgetOfExactType<PlanProvider>()
+        as PlanProvider;
     return provider._controller;
   }
 }
diff --git a/chapter_06/lib/repositories/in_memory_cache.dart b/chapter_06/lib/repositories/in_memory_cache.dart
index 6789c23..26342b6 100644
--- a/chapter_06/lib/repositories/in_memory_cache.dart
+++ b/chapter_06/lib/repositories/in_memory_cache.dart
@@ -1,13 +1,13 @@
 import 'repository.dart';
 
 class InMemoryCache implements Repository {
-  final _storage = Map<int, Model>();
+  final _storage = <int, Model>{};
 
   @override
   Model create() {
     final ids = _storage.keys.toList()..sort();
 
-    final id = (ids.length == 0) ? 1 : ids.last + 1;
+    final id = (ids.isEmpty) ? 1 : ids.last + 1;
 
     final model = Model(id: id);
     _storage[id] = model;
@@ -15,7 +15,7 @@ class InMemoryCache implements Repository {
   }
 
   @override
-  Model get(int id) {
+  Model? get(int id) {
     return _storage[id];
   }
 
diff --git a/chapter_06/lib/repositories/repository.dart b/chapter_06/lib/repositories/repository.dart
index 1defb57..a3ad207 100644
--- a/chapter_06/lib/repositories/repository.dart
+++ b/chapter_06/lib/repositories/repository.dart
@@ -1,10 +1,8 @@
-import 'package:flutter/foundation.dart';
-
 abstract class Repository {
   Model create();
 
   List<Model> getAll();
-  Model get(int id);
+  Model? get(int id);
   void update(Model item);
 
   void delete(Model item);
@@ -16,7 +14,7 @@ class Model {
   final Map data;
 
   const Model({
-    @required this.id,
+    required this.id,
     this.data = const {},
   });
-}
\ No newline at end of file
+}
diff --git a/chapter_06/lib/services/plan_services.dart b/chapter_06/lib/services/plan_services.dart
index 6f80cdb..fe05e67 100644
--- a/chapter_06/lib/services/plan_services.dart
+++ b/chapter_06/lib/services/plan_services.dart
@@ -3,7 +3,7 @@ import '../repositories/in_memory_cache.dart';
 import '../repositories/repository.dart';
 
 class PlanServices {
-  Repository _repository = InMemoryCache();
+  final Repository _repository = InMemoryCache();
 
   Plan createPlan(String name) {
     final model = _repository.create();
@@ -28,7 +28,10 @@ class PlanServices {
   }
 
   void addTask(Plan plan, String description) {
-    final id = plan.tasks.last?.id ?? 0 + 1;
+    int id = 0;
+    if (plan.tasks.isNotEmpty) {
+      id = plan.tasks.last.id + 1;
+    }
     final task = Task(id: id, description: description);
     plan.tasks.add(task);
     savePlan(plan);
diff --git a/chapter_06/lib/views/plan_screen.dart b/chapter_06/lib/views/plan_screen.dart
index 7dcbc7f..fc99bff 100644
--- a/chapter_06/lib/views/plan_screen.dart
+++ b/chapter_06/lib/views/plan_screen.dart
@@ -24,14 +24,25 @@ class _PlanScreenState extends State<PlanScreen> {
 
   @override
   Widget build(BuildContext context) {
-    // final plan = PlanProvider.of(context);
-    return Scaffold(
-        appBar: AppBar(title: Text('Master Plan')),
-        body: Column(children: <Widget>[
-          Expanded(child: _buildList()),
-          SafeArea(child: Text(plan.completenessMessage))
-        ]),
-        floatingActionButton: _buildAddTaskButton());
+    return WillPopScope(
+      onWillPop: () {
+        final controller = PlanProvider.of(context);
+        controller.savePlan(plan);
+        return Future.value(true);
+      },
+      child: Scaffold(
+        appBar: AppBar(
+          title: const Text('Master Plan'),
+        ),
+        body: Column(
+          children: [
+            Expanded(child: _buildList()),
+            SafeArea(child: Text(plan.completenessMessage))
+          ],
+        ),
+        floatingActionButton: _buildAddTaskButton(),
+      ),
+    );
   }
 
   @override
@@ -53,7 +64,6 @@ class _PlanScreenState extends State<PlanScreen> {
   }
 
   Widget _buildList() {
-    //final plan = PlanProvider.of(context);
     return ListView.builder(
       controller: scrollController,
       itemCount: plan.tasks.length,