diff --git a/example/android/app/build.gradle b/example/android/app/build.gradle
index a9339df..f987638 100644
--- a/example/android/app/build.gradle
+++ b/example/android/app/build.gradle
@@ -25,7 +25,7 @@ apply plugin: 'com.android.application'
 apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
 
 android {
-    compileSdkVersion 27
+    compileSdkVersion 28
 
     lintOptions {
         disable 'InvalidPackage'
diff --git a/example/android/gradle.properties b/example/android/gradle.properties
index 8bd86f6..4d3226a 100644
--- a/example/android/gradle.properties
+++ b/example/android/gradle.properties
@@ -1 +1,3 @@
 org.gradle.jvmargs=-Xmx1536M
+android.useAndroidX=true
+android.enableJetifier=true
\ No newline at end of file
diff --git a/lib/button_stagger_animation.dart b/lib/button_stagger_animation.dart
index 57ba1a7..3e73123 100644
--- a/lib/button_stagger_animation.dart
+++ b/lib/button_stagger_animation.dart
@@ -2,26 +2,26 @@ import 'package:flutter/material.dart';
 
 class ButtonStaggerAnimation extends StatelessWidget {
   // Animation fields
-  final AnimationController controller;
+  final  controller;
 
   // Display fields
-  final Color color;
-  final Color progressIndicatorColor;
-  final double progressIndicatorSize;
-  final BorderRadius borderRadius;
+  final Color? color;
+  final Color? progressIndicatorColor;
+  final double? progressIndicatorSize;
+  final BorderRadius? borderRadius;
   final double strokeWidth;
-  final Function(AnimationController) onPressed;
-  final Widget child;
+  final Function(AnimationController)? onPressed;
+  final Widget? child;
 
   ButtonStaggerAnimation({
-    Key key,
+    Key? key,
     this.controller,
     this.color,
     this.progressIndicatorColor,
     this.progressIndicatorSize,
     this.borderRadius,
     this.onPressed,
-    this.strokeWidth,
+    this.strokeWidth = 0.0,
     this.child,
   }) : super(key: key);
 
@@ -30,16 +30,16 @@ class ButtonStaggerAnimation extends StatelessWidget {
     return LayoutBuilder(builder: _progressAnimatedBuilder);
   }
 
-  Widget _buttonChild() {
-    if (controller.isAnimating) {
+  Widget? _buttonChild() {
+    if (controller!.isAnimating) {
       return Container();
-    } else if (controller.isCompleted) {
+    } else if (controller!.isCompleted) {
       return OverflowBox(
         maxWidth: progressIndicatorSize,
         maxHeight: progressIndicatorSize,
         child: CircularProgressIndicator(
           strokeWidth: strokeWidth,
-          valueColor: AlwaysStoppedAnimation<Color>(progressIndicatorColor),
+          valueColor: AlwaysStoppedAnimation<Color>(progressIndicatorColor!),
         ),
       );
     }
@@ -83,7 +83,7 @@ class ButtonStaggerAnimation extends StatelessWidget {
             color: color,
             child: _buttonChild(),
             onPressed: () {
-              this.onPressed(controller);
+              this.onPressed!(controller);
             },
           ),
         );
diff --git a/lib/progress_button.dart b/lib/progress_button.dart
index 7039236..cd905ea 100644
--- a/lib/progress_button.dart
+++ b/lib/progress_button.dart
@@ -1,7 +1,8 @@
 library progress_button;
 
 import 'package:flutter/material.dart';
-import 'package:progress_indicator_button/button_stagger_animation.dart';
+
+import 'button_stagger_animation.dart';
 
 class ProgressButton extends StatefulWidget {
   /// The background color of the button.
@@ -21,9 +22,9 @@ class ProgressButton extends StatefulWidget {
   ///
   /// This will grant access to its [AnimationController] so
   /// that the animation can be controlled based on the need.
-  final Function(AnimationController) onPressed;
+  final Function(AnimationController)? onPressed;
   /// The child to display on the button.
-  final Widget child;
+  final Widget? child;
 
   ProgressButton({
     @required this.child,
@@ -42,7 +43,7 @@ class ProgressButton extends StatefulWidget {
 
 class _ProgressButtonState extends State<ProgressButton>
     with TickerProviderStateMixin {
-  AnimationController _controller;
+  var _controller;
 
   @override
   void initState() {
diff --git a/pubspec.yaml b/pubspec.yaml
index cbc0389..367508d 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,11 +1,11 @@
 name: progress_indicator_button
 description: A simple button which can transform and show a progress indicator.
-version: 0.0.3
+version: 0.0.4
 author: Pascal Brosinski<pascal@brosinski.eu>
 homepage: https://github.com/PascalAC/progress_button
 
 environment:
-  sdk: ">=2.0.0-dev.68.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   flutter: