Provider null after pop - flutter

Question

I'm trying to use Provider with MVVM architecture, I have a very strange error with provider, on the main page there are 3 sections, banner, discounts and categories, when I change the quantity of goods in discounts, everything works, but after switching to a category and back, it already gives an error when I change the quantity of goods,
the data is not null, I think the problem is with the provider, the scheme is as follows:
discount section-> quantity of goods -> works
home-> category-> go back-> quantity of goods in discounts-> not working
Demo project
The following _CastError was thrown building MainPage(dirty, dependencies: [_InheritedProviderScope<AllGoodsViewModel?>, _LocalizationsScope-[GlobalKey#a9792], MediaQuery, _InheritedProviderScope<MainPageListViewModel?>], state: _MainPageState#eb568):
Null check operator used on a null value
The relevant error-causing widget was:
MainPage MainPage:file:///Users/.../lib/main.dart:93:21
When the exception was thrown, this was the stack:
#0 Element.widget (package:flutter/src/widgets/framework.dart:3229:31)
#1 debugCheckHasMediaQuery.<anonymous closure> (package:flutter/src/widgets/debug.dart:245:17)
#2 debugCheckHasMediaQuery (package:flutter/src/widgets/debug.dart:261:4)
#3 MediaQuery.of (package:flutter/src/widgets/media_query.dart:908:12)
#4 ScreenUtil.screenWidth (package:flutter_screenutil/src/screen_util.dart:148:37)
#5 ScreenUtil.scaleWidth (package:flutter_screenutil/src/screen_util.dart:167:28)
#6 ScreenUtil.setWidth (package:flutter_screenutil/src/screen_util.dart:182:41)
#7 SizeExtension.w (package:flutter_screenutil/src/size_extension.dart:9:32)
#8 _MainPageState.build (package:.../View/MainPage.dart:240:78)
#9 StatefulElement.build (package:flutter/src/widgets/framework.dart:4919:27)
#10 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4806:15)
#11 StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4977:11)
#12 Element.rebuild (package:flutter/src/widgets/framework.dart:4529:5)
#13 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2659:19)
#14 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:891:21)
#15 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:370:5)
#16 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1146:15)
#17 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1083:9)
#18 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:997:5)
#22 _invoke (dart:ui/hooks.dart:151:10)
#23 PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:308:5)
#24 _drawFrame (dart:ui/hooks.dart:115:31)
(elided 3 frames from dart:async)
ScreenUtilInitService
class ScreenUtilInitService z{
/// A helper widget that initializes [ScreenUtil]
ScreenUtilInitService({required this.builder, Key? key,}) : super(key: key);
final Widget Function(BuildContext) builder;
#override
Widget build(BuildContext context) {
return ScreenUtilInit(
designSize: Size(375, 812),
builder: (context, widget) => builder(context)
);
}
}
class MyApp extends StatefulWidget {
#override
State<MyApp> createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
bool _initialized = false;
bool _error = false;
Widget _present = SplashScreen();
void initializeFlutterFire() async {
try {
if (Platform.isAndroid) {
await AndroidInAppWebViewController.setWebContentsDebuggingEnabled(true);
}
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp();
await message();
await rootWidget();
setState(() {
_initialized = true;
});
} catch(e) {
setState(() {
_error = true;
});
}
}
Future<void> rootWidget() async {
final prefs = await SharedPreferences.getInstance();
final id = prefs.get("idAddress");
final logged = prefs.getBool("logged") ?? false;
if ((FirebaseAuth.instance.currentUser?.uid != null && logged) || id != null) {
setState(() {
_present = MainPage();
});
return;
} else {
setState(() {
_present = OfferPage();
});
return;
}
}
#override
void initState() {
initializeDB();
initializeFlutterFire();
super.initState();
listeners();
}
#override
Widget build(BuildContext context) {
if(_error) {
return MaterialApp(home:SplashScreen());
}
if (!_initialized) {
return MaterialApp(home:SplashScreen());
}
return RootPage().mainPage(present: _present);
}
}
and RootPage
class RootPage {
Widget mainPage({Widget? present}){
return MaterialApp(
initialRoute: '/',
routes: {
'/ProfilePage': (context) => ProfilePage(),
'/MainPage': (context) => MainPage(),
'/CartPage': (context) => CartPage(),
},
builder: (context, widget) {
return ScreenUtilInitService(
builder: (context) => widget!
);
},
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MultiProvider(
providers: [
ChangeNotifierProvider(
create: (context) => MainPageListViewModel(),
),
ChangeNotifierProvider(
create: (context) => CartViewModel(),
child: CartPage()
),
ChangeNotifierProvider(
create: (context) => AllGoodsViewModel(),
),
ChangeNotifierProvider(
create: (context) => GoodsViewModel(),
),
],
child: present != null ? present : MainPage(),
),
);
}
}
MainPage
#override
void initState() {
setting();
super.initState();
_scrollListener();
_notification();
}
#override
void dispose() {
_scrollController.removeListener(() { });
_scrollController.dispose();
NotificationCenter().unsubscribe('cart');
NotificationCenter().unsubscribe('address');
super.dispose();
}
void setting() async {
final cart = await SQFliteService.cart.getCount();
final address = await SQFliteService.location.current();
setState((){
_address = address;
showCart = cart == 0 ? false : true;
});
Provider.of<MainPageListViewModel>(context, listen: false).deliveryRequest() ;
Provider.of<MainPageListViewModel>(context, listen: false).fetchBanner();
Provider.of<MainPageListViewModel>(context, listen: false).fetchCategory();
Provider.of<AllGoodsViewModel>(context, listen: false).fetchSale();
Provider.of<MainPageListViewModel>(context, listen: false).fetchLaunchMessage(context);
}
void _scrollListener(){
_scrollController.addListener(() {
///setState is null after pop after category
// setState(() {
// _bottomOffSet = _scrollController.offset;
// });
});
}
void _notification(){
NotificationCenter().subscribe('cart', () async {
final result = await SQFliteService.cart.getCount();
setState(() {
showCart = result == 0 ? false : true;
print("update");
});
});
NotificationCenter().subscribe('address', () async {
final result = await SQFliteService.location.current();
setState((){
_address = result;
});
});
}
#override
Widget build(BuildContext context) {
final models = Provider.of<MainPageListViewModel>(context);
final sale = Provider.of<AllGoodsViewModel>(context);
final size = MediaQuery.of(context).size;
return Container(
child: CustomScrollView(
physics: AlwaysScrollableScrollPhysics(),
controller: _scrollController,
slivers: [
SliverAppBar(
...
),
SliverList(
delegate: SliverChildBuilderDelegate((context, index) => Container(
color: Colors.transparent,
child: Column(
children: [
bannerW(),
ViewSale(model: sale.goods),
SizedBox(height: 10),
ViewCategory(model: models.category)
]
),
),
childCount: 1
),
),
],
),
);
ViewSale
class _ViewSaleState extends State<ViewSale> {
#override
Widget build(BuildContext context) {
return Column(
children: [
Container(
margin: EdgeInsets.only(left: 16, right: 16, bottom: 15),
child: Align(
alignment: Alignment.centerLeft,
child: Text("Скидки", style: TextStyle(fontFamily: "Inter", fontWeight: FontWeight.w900, fontSize: 18))),
),
Container(
height: ((MediaQuery.of(context).size.width/2.4) - 21) + 71,
width: MediaQuery.of(context).size.width,
child: ListView.builder(
padding: EdgeInsets.symmetric(horizontal: 16),
shrinkWrap: true,
itemCount: widget.model.length,
scrollDirection: Axis.horizontal,
itemBuilder: (item, index) {
return ChangeNotifierProvider(
create: (context) => AllGoodsViewModel(),
child: ViewGoodsSale(model: widget.model[index])
);
}),
)
]
);
}
}
ViewSaleGoods
class _ViewGoodsSaleState extends State<ViewGoodsSale> {
GlobalKey _key = GlobalKey();
#override
void initState() {
super.initState();
WidgetsBinding.instance.addPostFrameCallback((_){
Provider.of<AllGoodsViewModel>(context, listen: false).setting(widget.model);
});
}
#override
Widget build(BuildContext context) {
final model = Provider.of<AllGoodsViewModel>(context);
final size = MediaQuery.of(context).size;
Widget inCart(){
return Container(
key: _key,
height: 31,
child: GestureDetector(
onPanDown: (details) {
Goods? item = widget.model;
RenderBox _cardBox = _key.currentContext!.findRenderObject() as RenderBox;
final localPosition = details.localPosition;
final localDx = localPosition.dx;
if (localDx <= _cardBox.size.width/2) {
Goods value = cart.firstWhere((element) => element.id == item.id);
if (item.optState == 0 ? value.orderCount <= 1 : value.orderCount <= value.opt!.count) {
setState(() {
context.read<AllGoodsViewModel>().setCountInCart(0);
final ind = cart.indexWhere((element) => element.id == item.id);
if (ind != -1) {
cart[ind].orderCount = 0;
SQFliteService.cart.delete(cart[ind].id);
cart.removeAt(ind);
}
});
} else {
model.haveItem(item: item, operation: item.optState == 0 ? -1 : (-1 * value.opt!.count));
}
} else {
model.haveItem(item: item, operation: item.optState == 0 ? 1 : item.count);
}
},
child: TextButton(
style: ButtonStyle(
backgroundColor: MaterialStateProperty.all(Design.appColor),
padding: MaterialStateProperty.all(EdgeInsets.symmetric(vertical: 8, horizontal: 10)),
shape: MaterialStateProperty.all(RoundedRectangleBorder(
borderRadius: BorderRadius.circular(10.0),
))
),
onPressed: (){},
child: Container(
child: RichText(
text: TextSpan(
text: "",
children:[
WidgetSpan(
alignment: PlaceholderAlignment.middle,
child: Icon(Icons.remove, size: 14, color: Colors.white),
),
TextSpan(
text: " ${widget.model.optState == 0 ? (widget.model.minPrice ?? widget.model.price) : widget.model.opt!.price} ₽ ",
style: TextStyle(
color: Colors.white,
fontSize: 14,
fontWeight: FontWeight.w500,
fontFamily: "Inter"
),
),
WidgetSpan(
alignment: PlaceholderAlignment.middle,
child: Icon(Icons.add, size: 14, color: Colors.white),
)
],
),
),
),
),
),// Your TextButton code goes here.
);
}
Widget noInCart(){
return Container(
key: _key,
height: 31,
child: TextButton(
style: ButtonStyle(
backgroundColor: MaterialStateProperty.all(model.orderBg),
padding: MaterialStateProperty.all(EdgeInsets.symmetric(vertical: 8, horizontal: 10)),
shape: MaterialStateProperty.all(RoundedRectangleBorder(
borderRadius: BorderRadius.circular(10.0),
))
),
onPressed: (){
Goods? item = widget.model;
model.haveItem(item: item, operation: item.optState == 0 ? 1 : item.count);
},
child: Container(
child: RichText(
text: TextSpan(
text: "${widget.model.optState == 0 ? widget.model.minPrice == null ? widget.model.price : widget.model.minPrice : widget.model.opt!.price} ₽ ",
style: TextStyle(
color: widget.model.minPrice != null ? Design.grey : Colors.black,
decoration: widget.model.optState == 0 && widget.model.minPrice != null ? TextDecoration.lineThrough : TextDecoration.none,
fontSize: 14,
fontWeight: FontWeight.w500,
fontFamily: "Inter"
),
children:[
TextSpan(
text: widget.model.minPrice == null ? "" : " ${widget.model.price} ₽",
style: TextStyle(
color: Colors.black,
decoration: TextDecoration.none,
fontSize: 14,
fontWeight: FontWeight.w500,
fontFamily: "Inter"
),
),
WidgetSpan(
alignment: PlaceholderAlignment.middle,
child: Icon(Icons.add, size: 14, color: Colors.black),
style: TextStyle(
color: Colors.black,
decoration: TextDecoration.none,
),
)
],
),
),
),
),
);
}
Widget card({required Size size}) {
return Container(
color: Colors.white,
width: (size.width/2.4) - 11,
margin: EdgeInsets.only(right: 10),
child: Column(
children: [
Stack(
children: [
Container(
height: (size.width/2.4) - 21,
padding: EdgeInsets.all(1),
decoration: BoxDecoration(
image: DecorationImage(
fit: BoxFit.contain,
image: NetworkImage(widget.model.images.first)
),
borderRadius: BorderRadius.all(Radius.circular(10.0)),
border: Border.all(
width: 1,
color: Design.lightGrey
),
),
),
Visibility(
visible: context.read<AllGoodsViewModel>().countInCart == 0 ? false : true,
child: Container(
height: (size.width/2.4) - 21,
decoration: BoxDecoration(
color: Colors.black.withOpacity(0.5),
borderRadius: BorderRadius.all(Radius.circular(10.0)),
),
child: Visibility(
visible: true,
child: Center(
child: model.orderCountText,
),
),
),
)
]
),
SizedBox(height: 5),
Align(
alignment: Alignment.centerLeft,
child: Container(
height: 29,
child: Text(widget.model.name,
maxLines: 2,
style: TextStyle(
fontSize: 12,
fontWeight: FontWeight.w700,
fontFamily: "Inter",
),
),
),
),
SizedBox(height: 6),
Align(
alignment: Alignment.centerLeft,
child: (context.read<AllGoodsViewModel>().countInCart == 0) ? noInCart() : inCart()
)
],
),
);
}
return card(size: size);
}
}
ViewCategory
class _ViewCategoryState extends State<ViewCategory> {
#override
Widget build(BuildContext context) {
return Container(
child: GridView.builder(
padding: EdgeInsets.all(16),
shrinkWrap: true,
physics: NeverScrollableScrollPhysics(),
gridDelegate: SliverGridDelegateWithFixedCrossAxisCount(
crossAxisCount: 2,
childAspectRatio: 1),
itemCount: widget.model.length,
itemBuilder: (context, index) {
return GestureDetector(
child: Stack(
children: [
Container(
decoration: BoxDecoration(
color: Design.lightGrey,
borderRadius: BorderRadius.all(Radius.circular(10.0)),
),
margin: EdgeInsets.only(right: (index.isOdd ? 0 : 5) , left: (index.isOdd ? 5 : 0), bottom: 10 ),
child: Image.network(widget.model[index].category?.url ?? "")
),
Positioned(
left: 10,
right: 10,
top: 8,
child: Text(widget.model[index].category?.name ?? "", style: TextStyle(fontFamily: "Inter", fontWeight: FontWeight.w500, fontSize: 12.sp)),
)
]
),
onTap: () async {
if (widget.model[index].category != null) {
final data = widget.model[index].category!;
final category = CategoryData(id: data.id, name: data.name);
if (data.tags == null) {
Navigator.of(context).push(
MaterialPageRoute(builder: (context) =>
ChangeNotifierProvider.value(value: AllGoodsViewModel(),
child: AllCategoryGoodsPage(category: category))
)
);
} else {
Navigator.of(context).push(
MaterialPageRoute(builder: (context) =>
ChangeNotifierProvider.value(value: GoodsViewModel(),
child: GoodsPage(category: category))
)
);
}
FirebaseAnalytics.instance.logEvent(name: "Category", parameters: null);
}
}
);
},
),
);
}
}

Answer 1

"Give a man a fish, feed him for a day. Teach a man to fish, feed him for a lifetime."
Thus, I try to explain what to do generally.
Firstly, look at the error stacktrace. You only provide the error message without stacktrace - thus no wonder nobody can see which line of code throws the error.
P.S. If you cannot find the stack trace, search or create a separate question (in some strange cases stack trace is not easy to find).
When you get the stack trace, you can read which line of code has this null pointer error, and which lines are calling that function. Suppose it is a.dart line 123 throwing the error, then look at that line and examine why that happens.
You may want to do "logging" or "debugging" to dig out further clues.
Then the bug should be found and solved easily. If not, try to create a minimal reproducible sample - your current sample is still very big and it is hard for people to dig and find bug for you :)

Answer 2

Your IDE will usually have great stack trace debugging. If you are using vscode, you can turn on the following two options for your error tracking.
If you're using Android Studio. Run -> View Breakpoints.

Answer 3

"Null check operator used on a null value" is a error you get when you use a bang operator (!) on a nullable instance which wasn't initialized.
In your "ViewSaleGoods" file you have it at -
if (item.optState == 0 ? value.orderCount <= 1 : value.orderCount <= value.opt!.count) { // Here .....
setState(() {
context.read<AllGoodsViewModel>().setCountInCart(0);
final ind = cart.indexWhere((element) => element.id == item.id);
if (ind != -1) {
cart[ind].orderCount = 0;
SQFliteService.cart.delete(cart[ind].id);
cart.removeAt(ind);
}
});
} else {
model.haveItem(item: item, operation: item.optState == 0 ? -1 : (-1 * value.opt!.count)); // here...
}
TextSpan(
text: " ${widget.model.optState == 0 ? (widget.model.minPrice ?? widget.model.price) : widget.model.opt!.price} ₽ ", // and here ....
style: TextStyle(
color: Colors.white,
fontSize: 14,
fontWeight: FontWeight.w500,
fontFamily: "Inter"
),
),
so anyone of those is turning out null so the error. Now to fix it you can use some ways like -
// Use variable and if check
var nullSafe;
if (value.opt != null) {
nullSafe = value.opt; // now you can use these variable instead
}
// Or use ?. and ??
value.opt?count ?? "some default value"
Hope it helps. 😄

Answer 4

I solved the problem, the error was not at all in the provider, but in ScreenUtilInitService I changed it to
class ScreenUtilInitService {
Widget build(Function(BuildContext) builder) {
return ScreenUtilInit(
designSize: Size(375, 812),
builder: (context, widget) => builder(context)
);
}
}
and removed ScreenUtilInitService from GoodsPage and AllCategoryGoodsPage so ScreenUtilInitService is not redefined as a widget, which resulted in the error