diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8627af6..79bf8bb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,9 @@
+## [4.0.0+1]
+* docs: Update CHANGELOG.md
+
+## [4.0.0]
+* refactor: Migrate to sound null safety 
+
 ## [3.0.5]
 * fix: cancel the use of `nullOk` attribute  
 
diff --git a/README.md b/README.md
index ce7b89b..c231888 100644
--- a/README.md
+++ b/README.md
@@ -49,9 +49,10 @@ Loading|Text|CustomWidget
 ### 🐮Getting started
 
 #### 1. add dependencies into you project pubspec.yaml file
-``` dart
+``` yaml
 dependencies:
-     bot_toast: ^3.0.5
+     bot_toast: ^3.0.5 
+#    bot_toast: ^4.0.0 #null safety
 ```
 
 #### 2. import BotToast lib
diff --git a/README_zh.md b/README_zh.md
index ec98932..ad238fa 100644
--- a/README_zh.md
+++ b/README_zh.md
@@ -53,9 +53,10 @@ Loading|Text|CustomWidget
 ### 🐮快速使用
 
 #### 1. pubspec.yaml文件里添加依赖
-``` dart
+``` yaml
 dependencies:
-     bot_toast: ^3.0.5
+     bot_toast: ^3.0.5 
+#    bot_toast: ^4.0.0 #null safety
 ```
 
 #### 2. 导入BotToast库
diff --git a/pubspec.yaml b/pubspec.yaml
index 627ed93..84092f1 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -3,7 +3,7 @@ description: A really easy to use flutter toast library.Easy to use and feature
 homepage: https://github.com/MMMzq/bot_toast
 email: vivask770@163.com
 
-version: 3.0.5
+version: 4.0.0+1
 
 environment:
   sdk: '>=2.12.0-0 <3.0.0'
diff --git a/test/widget_test.dart b/test/widget_test.dart
index 0e03972..f87545e 100644
--- a/test/widget_test.dart
+++ b/test/widget_test.dart
@@ -370,9 +370,9 @@ void main() {
 
 // ignore: must_be_immutable
 class TestApp extends StatelessWidget {
-  final VoidCallback callback;
-  final void Function(BuildContext ctx) contextCallback;
-  final Widget child;
+  final VoidCallback? callback;
+  final void Function(BuildContext ctx)? contextCallback;
+  final Widget? child;
 
   TestApp({this.callback, this.child, this.contextCallback});
 
@@ -391,17 +391,15 @@ class TestApp extends StatelessWidget {
           mainAxisSize: MainAxisSize.min,
           children: <Widget>[
             Builder(
-                builder: (ctx) =>
-                    FlatButton(
-                        key: ValueKey('main'),
-                        onPressed: () async {
-                          callback?.call();
-                          contextCallback?.call(ctx);
-                        },
-                        child: Text(''))),
-            child
-          ]
-            ..removeWhere((test) => test == null),
+                builder: (ctx) => FlatButton(
+                    key: ValueKey('main'),
+                    onPressed: () async {
+                      callback?.call();
+                      contextCallback?.call(ctx);
+                    },
+                    child: Text(''))),
+            if (child != null) child!
+          ],
         ),
       ),
     );