chore: modernize Flutter examples — null safety, latest APIs, Kotlin DSL Gradle (#148)

* updates

* Update Gradle wrapper script to fetch latest version online and allow --version override

* chore(analytics_integration): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build files to Kotlin DSL and bump Gradle wrapper.

* chore(animation_example): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build files to Kotlin DSL.

* chore(biometrics): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build files to Kotlin DSL.

* chore(bmi_calculator): test

test body

* chore(bottom_sheet): test

test body

* chore(custom_home_drawer): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build files to Kotlin DSL and bump Gradle wrapper.

* chore(covid19_mobile_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(dropdown_button): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(enabling_splash_screen): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(expense_planner): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(firebase_crash_reporting): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(firebase_google_authentication): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(flutter_date_pickers-master): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(getx_counter_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(google_signin): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(grid_layout): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(handling_routes): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(image_editor): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(image_from_network): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(infinite_list): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(load_local_image): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(load_local_json): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(lunch_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(navigation_drawer): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(news_memes_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(persist_key_value): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(push_notifications): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(save_data_locally_with_sqlite): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(scan_qr_code): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(simple_material_app): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(sliver_app_bar_example): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(stateful_widget): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(stateless_widgets): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(statless_counter_app): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(tic_tac_toe): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(tip_calculator): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(todo_list_using_provider): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(unit_testing): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_alert_dialog): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_bottom_nav_bar): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_custom_fonts): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_edittext): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_expansionpanel): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_firebase_db): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_gradient): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_http_get): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_interactiveviewer): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_listview): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_listwheelscrollview): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_platform_adaptive): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore(using_snackbar): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_stepper): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_tabs): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(using_theme): remove legacy nested android project

Delete the redundant nested android/ project directory from the old layout. Also update pubspec.yaml SDK constraints and dependencies.

* chore(view_pdf_file): migrate Gradle build scripts to Kotlin DSL

Convert Groovy build.gradle/settings.gradle to Kotlin DSL and bump Gradle wrapper.

* chore: ignore .trash directory

* chore: remove IDE configs and PR template

* chore(infinite_list): remove legacy nested android project

* refactor(infinite_list): migrate theme to ColorScheme API

* refactor(using_theme): migrate to ColorScheme API

* refactor(analytics_integration): migrate Firebase Analytics API

* refactor(firebase_google_authentication): migrate Google Sign-In API

* refactor(covid19_mobile_app): migrate theme and buttons to current API

* refactor(covid19_mobile_app): replace bezier_chart with fl_chart

* refactor(image_editor): migrate image plugins and button styles

* refactor(scan_qr_code): migrate mobile_scanner API

* refactor(lunch_app): migrate to null safety

* refactor(todo_list_using_provider): migrate to null safety

* refactor(unit_testing): migrate to null safety

* refactor(using_firebase_db): migrate Firebase Database API

* fix(update_gradle_wrapper): edit gradle-wrapper.properties directly

* refactor(push_notifications): migrate Firebase Messaging API

* refactor(using_http_get): migrate to null safety

* refactor(using_listview): migrate to null safety

* refactor(using_listwheelscrollview): migrate to null safety

* refactor(view_pdf_file): migrate to null safety

* refactor(bottom_sheet): migrate to null safety

* refactor(stateless_widgets): migrate to null safety

* chore: regenerate Gradle wrapper

* refactor: move maintenance scripts to Makefile

* chore: remove generated and IDE files

* fix: migrate image_editor to current plugins

* fix: migrate news_memes_app share to share_plus

* fix: migrate view_pdf_file to syncfusion pdf viewer

* fix: update deprecated APIs

* style: remove commented-out code

covid19_mobile_app/lib/screens/countrylist.dart

@@ -10,7 +10,7 @@ import 'package:folding_cell/folding_cell.dart';
 class CountryList extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
-    List<dynamic> results;
+    List<dynamic> results = [];
     return Scaffold(
       drawer: DrawerWidget(),
       appBar: AppBar(
@@ -48,14 +48,15 @@ class CountryList extends StatelessWidget {
               return Container(
                 color: Color(0xFF2e282a),
                 alignment: Alignment.topCenter,
-                child: SimpleFoldingCell(
+                child: SimpleFoldingCell.create(
                   key: _foldingCellKey,
                   frontWidget: Builder(
                     builder: (BuildContext context) {
                       return GestureDetector(
                         onTap: () {
-                          SimpleFoldingCellState foldingCellState =
-                              context.findAncestorStateOfType();
+                          SimpleFoldingCellState? foldingCellState =
+                              context.findAncestorStateOfType<
+                                  SimpleFoldingCellState>();
                           foldingCellState?.toggleFold();
                         },
                         child: Container(
@@ -68,17 +69,21 @@ class CountryList extends StatelessWidget {
                       );
                     },
                   ),
-                  innerTopWidget: buildInnerTopWidget(
-                    results[index]['country'],
-                    results[index]['todayCases'].toString(),
-                    results[index]['deaths'].toString(),
-                    results[index]['todayDeaths'].toString(),
-                    results[index]['recovered'].toString(),
-                    results[index]['critical'].toString(),
-                    results[index]['casesPerOneMillion'].toString(),
+                  innerWidget: Column(
+                    children: [
+                      buildInnerTopWidget(
+                        results[index]['country'],
+                        results[index]['todayCases'].toString(),
+                        results[index]['deaths'].toString(),
+                        results[index]['todayDeaths'].toString(),
+                        results[index]['recovered'].toString(),
+                        results[index]['critical'].toString(),
+                        results[index]['casesPerOneMillion'].toString(),
+                      ),
+                      buildInnerBottomWidget(
+                          results[index]['cases'].toString()),
+                    ],
                   ),
-                  innerBottomWidget: buildInnerBottomWidget(
-                      results[index]['cases'].toString()),
                   cellSize: Size(MediaQuery.of(context).size.width, 125),
                   padding: EdgeInsets.all(15),
                   animationDuration: Duration(milliseconds: 300),