fix #196, #209

@@ -50,7 +50,7 @@ class MyApp extends StatelessWidget {
     return ScreenUtilInit(
       designSize: Size(360, 690),
       allowFontScaling: false,
-      child: MaterialApp(
+      builder: () => MaterialApp(
         ...
       ),
     );

@@ -58,7 +58,7 @@ class MyApp extends StatelessWidget {
     return ScreenUtilInit(
       designSize: Size(360, 690),
       allowFontScaling: false,
-      child: MaterialApp(
+      builder: () => MaterialApp(
         ...
       ),
     );

@@ -54,7 +54,7 @@ class MyApp extends StatelessWidget {
     return ScreenUtilInit(
       designSize: Size(360, 690),
       allowFontScaling: false,
-      child: MaterialApp(
+      builder: () => MaterialApp(
         ...
       ),
     );

@@ -10,7 +10,7 @@ class MyApp extends StatelessWidget {
     return ScreenUtilInit(
       designSize: Size(360, 690),
       allowFontScaling: false,
-      child: MaterialApp(
+      builder: () => MaterialApp(
         debugShowCheckedModeBanner: false,
         title: 'Flutter_ScreenUtil',
         theme: ThemeData(

@@ -10,7 +10,7 @@ class MyApp extends StatelessWidget {
     return ScreenUtilInit(
       designSize: Size(750, 1334),
       allowFontScaling: false,
-      child: MaterialApp(
+      builder: () => MaterialApp(
         debugShowCheckedModeBanner: false,
         title: 'Flutter_ScreenUtil',
         theme: ThemeData(

@@ -5,13 +5,13 @@ import 'screenutil.dart';
 class ScreenUtilInit extends StatelessWidget {
   /// A helper widget that initializes [ScreenUtil]
   ScreenUtilInit({
-    required this.child,
+    required this.builder,
     this.designSize = ScreenUtil.defaultSize,
     this.allowFontScaling = false,
     Key? key,
   }) : super(key: key);
 
-  final Widget child;
+  final Widget Function() builder;
 
   /// The [Size] of the device in the design draft, in dp
   final Size designSize;
@@ -31,7 +31,7 @@ class ScreenUtilInit extends StatelessWidget {
             allowFontScaling: allowFontScaling,
           );
         }
-        return child;
+        return builder();
       },
     );
   }