Null check operator used on a null value || Flutter - flutter

Question

The null check operator used on a null value error comes up sometimes on the UI otherwise a circular progress indicator shows up.
the error shows up in the 266 line.
class Orders extends StatefulWidget {
#override
_OrdersState createState() => _OrdersState();
}
class _OrdersState extends State<Orders> {
OrdersModel? order;
OrdersModel? order2;
void modelData() async {
order2 = await ordersModel();
setState(() {
order = order2;
});
}
#override
void initState() {
modelData();
_futureOrdersModel = ordersModel();
// ordersModel();
super.initState();
}
Widget categories() {
return buildFutureBuilder();
}
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: primaryColor,
body: Container(
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: [
);
}
late Future<OrdersModel>? _futureOrdersModel;
FutureBuilder<OrdersModel> buildFutureBuilder() {
return FutureBuilder<OrdersModel>(
future: _futureOrdersModel,
builder: (context, snapshot) {
if (snapshot.hasData) {
bool? checkedValue =
order?.data.attributes.totalBills[0].manualBillCompletion
?? true;
return ListView.builder(
padding: EdgeInsets.zero,
itemCount: snapshot.data?.data.attributes.totalBills.length ?? 0,
itemBuilder: (BuildContext context, int index) {
return InkWell(
onTap: () {
Navigator.push(context,
MaterialPageRoute(builder: (context) => Review()));
},
],
),
decoration: BoxDecoration(
color: Colors.white,
borderRadius:
BorderRadius.vertical(top: Radius.circular(15))),
),
); //categoryCard1();
},
);
} else if (snapshot.hasError) {
return Text('${snapshot.error}');
}
return Column(
children: [
Center(child: const CircularProgressIndicator()),
],
);
},
);
}
}
check my console error to get it more clear of my errors
in console, it shows error due to bottomNavigation here is the code for that also

Answer 1

this null check error because I am using bang operator (!) to suppress the error. But on run time when my app runs this error shows up.
example:- How to use ? and ?? operator properly
String? name; //Here name is a nullable variable
void main()
{
var len = name?.length?? 0;
}
if the name is null then the value will be appointed 0.
so in the itemCount of the listview.builder I used ?? operator like this.
itemCount: customer?.data.length?? 0,