Flutter Firestore StreamBuilder getting null value initially - firebase

Question

I'm trying to get data from firestore by using stream builder method. initially, when I run the app I don't get any data from firebase. I'm getting
Invalid value: Valid value range is empty: 0.
once I reload app the data is available
please find the code bellow
Widget build(BuildContext context) {
return StreamBuilder(
stream: Firestore.instance
.collection('users/')
.where('uid', isEqualTo: _userUID)
.snapshots(),
builder: (BuildContext context, userSnapshot) {
if (!userSnapshot.hasData) return WidgetFunctions().loadingWidget();
return StreamBuilder(
stream: Firestore.instance
.collection('products')
.where('latest', isEqualTo: true)
.snapshots(),
builder: (cuserSnapshotontext, snapshot) {
if (!snapshot.hasData) return WidgetFunctions().loadingWidget();
if (snapshot.data.documents.length == 0)
return const Center(
child: Text(
"Not Available",
style: TextStyle(fontSize: 30.0, color: Colors.grey),
),
);
if (!userSnapshot.data.documents[0]['productViewPermission']) {
print('place6');
return const Center(
child: Text(
'You dont have permission to view products \n please contect us',
style: TextStyle(
fontSize: 18.0,
color: Colors.red,
fontWeight: FontWeight.bold),
));
}
return GridView.builder(
shrinkWrap: true,
scrollDirection: Axis.vertical,
itemCount: snapshot.data.documents.length,
gridDelegate: new SliverGridDelegateWithFixedCrossAxisCount(
crossAxisCount: 2,
),
itemBuilder: (BuildContext context, int index) {
return SingleProduct(
productCatagory: snapshot.data.documents[index]
['productCatogary'],
productName: snapshot.data.documents[index]
['productName'],
imageURL: snapshot.data.documents[index]['imageURLS']
[0],
price: userSnapshot.data.documents[0]
['priceViewpermission']
? snapshot.data.documents[index]['price'].toDouble()
: "To view price please contect us",
discountBool: snapshot.data.documents[index]
['discount'],
discountValue: snapshot.data.documents[index]
['discountValue'],
index: index,
description: snapshot.data.documents[index]
['description'],
make: snapshot.data.documents[index]['make'],
karat: snapshot.data.documents[index]['karat'],
waight:
snapshot.data.documents[index]['waight'].toDouble(),
condition: snapshot.data.documents[index]['condition'],
populer: snapshot.data.documents[index]['populer'],
isAvailable: snapshot.data.documents[index]
['isAvailable']);
},
);
});
});
}
I/flutter ( 3686): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
I/flutter ( 3686): The following RangeError was thrown building StreamBuilder<QuerySnapshot>(dirty, state:
I/flutter ( 3686): _StreamBuilderBaseState<QuerySnapshot, AsyncSnapshot<QuerySnapshot>>#02fde):
I/flutter ( 3686): RangeError (index): Invalid value: Valid value range is empty: 0
I/flutter ( 3686): When the exception was thrown, this was the stack:
I/flutter ( 3686): #0 List.[] (dart:core-patch/growable_array.dart:145:60)
I/flutter ( 3686): #1 _PopularProductsContainerState.build.<anonymous closure>.<anonymous closure> (package:thaya_factory/Components/HomePageComponents/CategoryComponent/LatestProductsComponent.dart:54:49)
I/flutter ( 3686): #2 StreamBuilder.build (package:flutter/src/widgets/async.dart:425:74)
I/flutter ( 3686): #3 _StreamBuilderBaseState.build (package:flutter/src/widgets/async.dart:125:48)
I/flutter ( 3686): #4 StatefulElement.build (package:flutter/src/widgets/framework.dart:4012:27)
I/flutter ( 3686): #5 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3924:15)
I/flutter ( 3686): #6 Element.rebuild (package:flutter/src/widgets/framework.dart:3721:5)
I/flutter ( 3686): #7 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2340:33)
I/flutter ( 3686): #8 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:700:20)
I/flutter ( 3686): #9 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&SemanticsBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:285:5)
I/flutter ( 3686): #10 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1016:15)
I/flutter ( 3686): #11 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:958:9)
I/flutter ( 3686): #12 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:874:5)
I/flutter ( 3686): #16 _invoke (dart:ui/hooks.dart:236:10)
I/flutter ( 3686): #17 _drawFrame (dart:ui/hooks.dart:194:3)
I/flutter ( 3686): (elided 3 frames from package dart:async)
I/flutter ( 3686): ═══════════════════════
Please help me to fix the issue
Thanks in advance

Answer 1

Well first of all, this block of code has invalid syntax:
if (snapshot.data.documents.length == 0)
return const Center(
child: Text(
"Not Available",
style: TextStyle(fontSize: 30.0, color: Colors.grey),
),
);
You forgot the opening and closing brackets on the if statement.
And secondly, try an else if on the second if statement, because now, when snapshot.data.documents.length returns null, it will still check for !userSnapshot.data.documents[0], though the list is empty, which is the cause for your RangeError.
So try this:
if (snapshot.data.documents.length == 0) {
return const Center(
child: Text(
"Not Available",
style: TextStyle(fontSize: 30.0, color: Colors.grey),
),
);
} else if (!userSnapshot.data.documents[0]['productViewPermission']) {
print('place6');
return const Center(
child: Text(
'You dont have permission to view products \n please contect us',
style: TextStyle(
fontSize: 18.0,
color: Colors.red,
fontWeight: FontWeight.bold),
)
);
}

Answer 2

This is because itemCount of GridView is accessing no data snaphsot.
Try doing
itemCount = snapshot.data== null ? 0 : snapshot.data.documents.length;