diff --git a/.packages b/.packages
index 087a87e..db3f6b0 100644
--- a/.packages
+++ b/.packages
@@ -1,29 +1,28 @@
-# Generated by pub on 2019-10-06 15:58:05.772599.
-archive:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/archive-2.0.10/lib/
-args:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/args-1.5.2/lib/
-async:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/async-2.3.0/lib/
-boolean_selector:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/boolean_selector-1.0.5/lib/
-charcode:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/charcode-1.1.2/lib/
-collection:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/collection-1.14.11/lib/
-convert:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/convert-2.1.1/lib/
-crypto:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/crypto-2.1.3/lib/
-flutter:file:///Users/chokyungjin/flutter/packages/flutter/lib/
-flutter_test:file:///Users/chokyungjin/flutter/packages/flutter_test/lib/
-image:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/image-2.1.4/lib/
-matcher:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/matcher-0.12.5/lib/
-meta:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/meta-1.1.7/lib/
-path:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/path-1.6.4/lib/
-pedantic:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/pedantic-1.8.0+1/lib/
-petitparser:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/petitparser-2.4.0/lib/
-quiver:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/quiver-2.0.5/lib/
-sky_engine:file:///Users/chokyungjin/flutter/bin/cache/pkg/sky_engine/lib/
-source_span:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/source_span-1.5.5/lib/
-stack_trace:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/stack_trace-1.9.3/lib/
-stream_channel:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/stream_channel-2.0.0/lib/
-string_scanner:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/string_scanner-1.0.5/lib/
-term_glyph:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/term_glyph-1.1.0/lib/
-test_api:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/test_api-0.2.5/lib/
-typed_data:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/typed_data-1.1.6/lib/
-vector_math:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/vector_math-2.0.8/lib/
-xml:file:///Users/chokyungjin/flutter/.pub-cache/hosted/pub.dartlang.org/xml-3.5.0/lib/
+# This file is deprecated. Tools should instead consume 
+# `.dart_tools/package_config.json`.
+# 
+# For more info see: https://dart.dev/go/dot-packages-deprecation
+# 
+# Generated by pub on 2021-03-11 16:54:14.009224.
+async:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/async-2.5.0/lib/
+boolean_selector:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/boolean_selector-2.1.0/lib/
+characters:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/characters-1.1.0/lib/
+charcode:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/charcode-1.2.0/lib/
+clock:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/clock-1.1.0/lib/
+collection:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/collection-1.15.0/lib/
+fake_async:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/fake_async-1.2.0/lib/
+flutter:file:///C:/flutter/packages/flutter/lib/
+flutter_test:file:///C:/flutter/packages/flutter_test/lib/
+matcher:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/matcher-0.12.10/lib/
+meta:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/meta-1.3.0/lib/
+path:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/path-1.8.0/lib/
+sky_engine:file:///C:/flutter/bin/cache/pkg/sky_engine/lib/
+source_span:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/source_span-1.8.0/lib/
+stack_trace:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/stack_trace-1.10.0/lib/
+stream_channel:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/stream_channel-2.1.0/lib/
+string_scanner:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/string_scanner-1.1.0/lib/
+term_glyph:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/term_glyph-1.2.0/lib/
+test_api:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/test_api-0.2.19/lib/
+typed_data:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/typed_data-1.3.0/lib/
+vector_math:file:///C:/flutter/.pub-cache/hosted/pub.dartlang.org/vector_math-2.1.0/lib/
 open_appstore:lib/
diff --git a/example/.flutter-plugins-dependencies b/example/.flutter-plugins-dependencies
new file mode 100644
index 0000000..a8629ea
--- /dev/null
+++ b/example/.flutter-plugins-dependencies
@@ -0,0 +1 @@
+{"info":"This is a generated file; do not edit or check into version control.","plugins":{"ios":[{"name":"open_appstore","path":"C:\\\\Users\\\\ChangJoo Park\\\\Documents\\\\GitHub\\\\flutter_open_appstore\\\\","dependencies":[]}],"android":[{"name":"open_appstore","path":"C:\\\\Users\\\\ChangJoo Park\\\\Documents\\\\GitHub\\\\flutter_open_appstore\\\\","dependencies":[]}],"macos":[],"linux":[],"windows":[],"web":[]},"dependencyGraph":[{"name":"open_appstore","dependencies":[]}],"date_created":"2021-03-11 16:57:48.762501","version":"2.0.1"}
\ No newline at end of file
diff --git a/example/pubspec.lock b/example/pubspec.lock
index deb8e3e..086cfa5 100644
--- a/example/pubspec.lock
+++ b/example/pubspec.lock
@@ -1,69 +1,62 @@
 # Generated by pub
 # See https://dart.dev/tools/pub/glossary#lockfile
 packages:
-  archive:
-    dependency: transitive
-    description:
-      name: archive
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.0.10"
-  args:
-    dependency: transitive
-    description:
-      name: args
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "1.5.2"
   async:
     dependency: transitive
     description:
       name: async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.3.0"
+    version: "2.5.0"
   boolean_selector:
     dependency: transitive
     description:
       name: boolean_selector
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.0.5"
-  charcode:
+    version: "2.1.0"
+  characters:
     dependency: transitive
     description:
-      name: charcode
+      name: characters
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.2"
-  collection:
+    version: "1.1.0"
+  charcode:
     dependency: transitive
     description:
-      name: collection
+      name: charcode
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.14.11"
-  convert:
+    version: "1.2.0"
+  clock:
     dependency: transitive
     description:
-      name: convert
+      name: clock
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.1"
-  crypto:
+    version: "1.1.0"
+  collection:
     dependency: transitive
     description:
-      name: crypto
+      name: collection
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.3"
+    version: "1.15.0"
   cupertino_icons:
     dependency: "direct main"
     description:
       name: cupertino_icons
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.1.2"
+    version: "1.0.2"
+  fake_async:
+    dependency: transitive
+    description:
+      name: fake_async
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.2.0"
   flutter:
     dependency: "direct main"
     description: flutter
@@ -74,62 +67,34 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
-  image:
-    dependency: transitive
-    description:
-      name: image
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.1.4"
   matcher:
     dependency: transitive
     description:
       name: matcher
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.12.5"
+    version: "0.12.10"
   meta:
     dependency: transitive
     description:
       name: meta
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.7"
+    version: "1.3.0"
   open_appstore:
     dependency: "direct dev"
     description:
       path: ".."
       relative: true
     source: path
-    version: "1.0.0"
+    version: "1.0.2"
   path:
     dependency: transitive
     description:
       name: path
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.6.4"
-  pedantic:
-    dependency: transitive
-    description:
-      name: pedantic
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "1.8.0+1"
-  petitparser:
-    dependency: transitive
-    description:
-      name: petitparser
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.4.0"
-  quiver:
-    dependency: transitive
-    description:
-      name: quiver
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.0.5"
+    version: "1.8.0"
   sky_engine:
     dependency: transitive
     description: flutter
@@ -141,62 +106,55 @@ packages:
       name: source_span
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.5.5"
+    version: "1.8.0"
   stack_trace:
     dependency: transitive
     description:
       name: stack_trace
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.9.3"
+    version: "1.10.0"
   stream_channel:
     dependency: transitive
     description:
       name: stream_channel
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.0.0"
+    version: "2.1.0"
   string_scanner:
     dependency: transitive
     description:
       name: string_scanner
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.0.5"
+    version: "1.1.0"
   term_glyph:
     dependency: transitive
     description:
       name: term_glyph
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0"
+    version: "1.2.0"
   test_api:
     dependency: transitive
     description:
       name: test_api
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.2.5"
+    version: "0.2.19"
   typed_data:
     dependency: transitive
     description:
       name: typed_data
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.6"
+    version: "1.3.0"
   vector_math:
     dependency: transitive
     description:
       name: vector_math
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.0.8"
-  xml:
-    dependency: transitive
-    description:
-      name: xml
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "3.5.0"
+    version: "2.1.0"
 sdks:
-  dart: ">=2.4.0 <3.0.0"
+  dart: ">=2.12.0 <3.0.0"
diff --git a/example/pubspec.yaml b/example/pubspec.yaml
index 4bf4d20..fa8969f 100644
--- a/example/pubspec.yaml
+++ b/example/pubspec.yaml
@@ -3,7 +3,7 @@ description: Demonstrates how to use the open_appstore plugin.
 publish_to: 'none'
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   flutter:
@@ -11,7 +11,7 @@ dependencies:
 
   # The following adds the Cupertino Icons font to your application.
   # Use with the CupertinoIcons class for iOS style icons.
-  cupertino_icons: ^0.1.2
+  cupertino_icons: ^1.0.2
 
 dev_dependencies:
   flutter_test:
diff --git a/example/test/widget_test.dart b/example/test/widget_test.dart
index 80c7366..24b670c 100644
--- a/example/test/widget_test.dart
+++ b/example/test/widget_test.dart
@@ -19,7 +19,7 @@ void main() {
     expect(
       find.byWidgetPredicate(
         (Widget widget) => widget is Text &&
-                           widget.data.startsWith('Running on:'),
+                           widget.data!.startsWith('Running on:'),
       ),
       findsOneWidget,
     );
diff --git a/lib/open_appstore.dart b/lib/open_appstore.dart
index f06499b..7ff60e5 100644
--- a/lib/open_appstore.dart
+++ b/lib/open_appstore.dart
@@ -6,12 +6,12 @@ class OpenAppstore {
   static const MethodChannel _channel =
       const MethodChannel('flutter.moum.open_appstore');
 
-  static Future<String> get platformVersion async {
-    final String version = await _channel.invokeMethod('getPlatformVersion');
+  static Future<String?> get platformVersion async {
+    final String? version = await _channel.invokeMethod('getPlatformVersion');
     return version;
   }
 
-  static void launch({String androidAppId, String iOSAppId}) async {
+  static void launch({String? androidAppId, String? iOSAppId}) async {
     await _channel.invokeMethod(
         'openappstore', {'android_id': androidAppId, 'ios_id':iOSAppId});
   }
diff --git a/pubspec.lock b/pubspec.lock
index a14b07d..49a0ce6 100644
--- a/pubspec.lock
+++ b/pubspec.lock
@@ -1,62 +1,55 @@
 # Generated by pub
 # See https://dart.dev/tools/pub/glossary#lockfile
 packages:
-  archive:
-    dependency: transitive
-    description:
-      name: archive
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.0.10"
-  args:
-    dependency: transitive
-    description:
-      name: args
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "1.5.2"
   async:
     dependency: transitive
     description:
       name: async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.3.0"
+    version: "2.5.0"
   boolean_selector:
     dependency: transitive
     description:
       name: boolean_selector
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.0.5"
+    version: "2.1.0"
+  characters:
+    dependency: transitive
+    description:
+      name: characters
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.1.0"
   charcode:
     dependency: transitive
     description:
       name: charcode
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.2"
-  collection:
+    version: "1.2.0"
+  clock:
     dependency: transitive
     description:
-      name: collection
+      name: clock
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.14.11"
-  convert:
+    version: "1.1.0"
+  collection:
     dependency: transitive
     description:
-      name: convert
+      name: collection
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.1"
-  crypto:
+    version: "1.15.0"
+  fake_async:
     dependency: transitive
     description:
-      name: crypto
+      name: fake_async
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.1.3"
+    version: "1.2.0"
   flutter:
     dependency: "direct main"
     description: flutter
@@ -67,55 +60,27 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
-  image:
-    dependency: transitive
-    description:
-      name: image
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.1.4"
   matcher:
     dependency: transitive
     description:
       name: matcher
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.12.5"
+    version: "0.12.10"
   meta:
     dependency: transitive
     description:
       name: meta
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.7"
+    version: "1.3.0"
   path:
     dependency: transitive
     description:
       name: path
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.6.4"
-  pedantic:
-    dependency: transitive
-    description:
-      name: pedantic
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "1.8.0+1"
-  petitparser:
-    dependency: transitive
-    description:
-      name: petitparser
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.4.0"
-  quiver:
-    dependency: transitive
-    description:
-      name: quiver
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "2.0.5"
+    version: "1.8.0"
   sky_engine:
     dependency: transitive
     description: flutter
@@ -127,62 +92,55 @@ packages:
       name: source_span
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.5.5"
+    version: "1.8.0"
   stack_trace:
     dependency: transitive
     description:
       name: stack_trace
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.9.3"
+    version: "1.10.0"
   stream_channel:
     dependency: transitive
     description:
       name: stream_channel
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.0.0"
+    version: "2.1.0"
   string_scanner:
     dependency: transitive
     description:
       name: string_scanner
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.0.5"
+    version: "1.1.0"
   term_glyph:
     dependency: transitive
     description:
       name: term_glyph
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.0"
+    version: "1.2.0"
   test_api:
     dependency: transitive
     description:
       name: test_api
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.2.5"
+    version: "0.2.19"
   typed_data:
     dependency: transitive
     description:
       name: typed_data
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "1.1.6"
+    version: "1.3.0"
   vector_math:
     dependency: transitive
     description:
       name: vector_math
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.0.8"
-  xml:
-    dependency: transitive
-    description:
-      name: xml
-      url: "https://pub.dartlang.org"
-    source: hosted
-    version: "3.5.0"
+    version: "2.1.0"
 sdks:
-  dart: ">=2.4.0 <3.0.0"
+  dart: ">=2.12.0 <3.0.0"
diff --git a/pubspec.yaml b/pubspec.yaml
index fa854fc..ddf873a 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -8,7 +8,7 @@ authors:
 homepage: https://github.com/flutter-moum/flutter_open_appstore
 
 environment:
-  sdk: ">=2.1.0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   flutter: