Null check operator used on a null value

[sha-flutterdev]

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following _CastError was thrown building Builder(dirty):
Null check operator used on a null value

The relevant error-causing widget was:
MaterialApp file:///E:/AS%20PROJECTS/FLUTTER_APPS/flutter_application_day3/flutter_app_demo_responsive/lib/main.dart:61:12
When the exception was thrown, this was the stack:
#0 MyApp1.build. (package:flutter_app_demo_responsive/main.dart:63:56)
#1 _MaterialAppState._materialBuilder. (package:flutter/src/material/app.dart:815:39)
#2 Builder.build (package:flutter/src/widgets/basic.dart:7798:48)
#3 StatelessElement.build (package:flutter/src/widgets/framework.dart:4648:28)
#4 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4574:15)
...
════════════════════════════════════════════════════════════════════════════════════════════════════
Code:
return MaterialApp(
builder: (context, widget) => ResponsiveWrapper.builder(
BouncingScrollWrapper.builder(context, widget!),
maxWidth: 1200,
minWidth: 450,
defaultScale: true,
breakpoints: [
ResponsiveBreakpoint.resize(450, name: MOBILE),
ResponsiveBreakpoint.autoScale(800, name: TABLET),
ResponsiveBreakpoint.autoScale(1000, name: TABLET),
ResponsiveBreakpoint.resize(1200, name: DESKTOP),
ResponsiveBreakpoint.autoScale(2460, name: "4K"),
],
background: Container(child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Padding(
padding: const EdgeInsets.all(10.0),
child: Container(
child:Text('Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s.',style: TextStyle(fontSize: 14, fontFamily: 'Roboto', color: Colors.black,),
),
),
),
Container(
child:Text('Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s.',style: TextStyle(fontSize: 16,fontWeight: FontWeight.bold,fontFamily: 'Roboto',color: Colors.black,),),
),
Container(
child:Text('Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s.',style: TextStyle(fontSize: 18, color: Colors.black,fontFamily: 'Roboto',fontWeight: FontWeight.w300),),
),
],
),
),
),
debugShowCheckedModeBanner: false,
);