我在我的主页上添加了三个 streamBuilder，所有 StreamBuilders 从云 Firestore 获取不同的数据，但我收到 'The getter 'length' was called on null in flutter' 。我试图在互联网上寻找解决方案，但都是徒劳的。我收到此错误：
════════ Exception caught by widgets library ═══════════════════════════════════
The getter 'length' was called on null.
Receiver: null
Tried calling: length
The relevant error-causing widget was
StreamBuilder<List<Order>>
lib/…/home/home.dart:52
这是我的家。dart：
import 'package:flutter/material.dart';
import 'package:merza/models/orders.dart';
import 'package:merza/models/user.dart';
import 'package:merza/screens/home/items.dart';
import 'package:merza/screens/home/orders/delivered_order.dart';
import 'package:merza/screens/home/orders/order_status.dart';
import 'package:merza/screens/user/add_order.dart';
import 'package:merza/services/auth.dart';
import 'package:merza/services/databse.dart';
import 'package:merza/shared/grid.dart';
import 'package:intl/intl.dart';
import 'package:merza/shared/menus.dart';
import 'package:merza/shared/splash_screen.dart';
import 'package:provider/provider.dart';
import 'package:merza/screens/home/orders/orders_list.dart';
import 'package:firebase_auth/firebase_auth.dart';
import 'package:cloud_firestore/cloud_firestore.dart';
import 'orders/orders_tile.dart';
class Home extends StatelessWidget {
//Auth
AuthService _auth = AuthService();
//Database
Database _db = Database();
@override
Widget build(BuildContext context) {
final user = Provider.of<User>(context);
if(user.uid == null){
Navigator.pushNamedAndRemoveUntil(context, "/home", (r) => false);
}else{
return StreamBuilder<UserData>(
stream: Database(uid: user.uid).userData,
builder: (context, snapshot) {
if (snapshot.hasData) {
UserData userData = snapshot.data;
//Streambuilder for delivered orders
return StreamBuilder<List<Order>>(
stream: Database(receiver_nrc: userData.nrc, paramOne: 'payment_status', paramValue: 'unpaid').dataFetch,
builder: (context, snapshot2) {
String count_unpaid = snapshot2.data.length.toString();
if (snapshot2.hasData) {
return StreamBuilder <List<Order>>(
stream: Database(receiver_nrc: userData.nrc, paramOne: 'order_status', paramValue: 'delivered').dataFetch,
builder: (context, snapshot3) {
String count_awaiting = snapshot3.data.length.toString();
if (snapshot3.hasData) {
return StreamProvider<List<Order>>.value(
value: Database(receiver_nrc: userData.nrc).orders,
child: Scaffold(
appBar: LoggedBar(title: 'Merza',),
body: SingleChildScrollView(
child: Column(
children: [
Container(), // Required some widget in between to float AppBar
Container( // To take AppBar Size only
child: AppBar(
elevation: 0.0,
backgroundColor: Colors.white,
leading: IconButton(icon: Icon(Icons.home), color: Colors.green[900], onPressed: (){
Navigator.push(context, MaterialPageRoute(builder: (context) => Home()));
},),
primary: false,
title: TextField(
decoration: InputDecoration(
hintText: "Search",
border: InputBorder.none,
hintStyle: TextStyle(color: Colors.green[900]))),
actions: <Widget>[
IconButton(
icon: Icon(Icons.search, color: Colors.green[900]), onPressed: () {},),
],
),
),
Container(
color: Colors.green,
padding: EdgeInsets.symmetric(vertical: 20.0, horizontal: 20.0),
child: Text('A TRADING ZONE THAT PROMOTES SAFETY TRADING AND SECURE DIGITAL TRANSACTIONS', style: TextStyle(fontWeight: FontWeight.bold, fontSize: 14, color: Colors.white)),
), Container(
margin: EdgeInsets.symmetric(vertical: 10.0, horizontal: 20.0),
child: Row(
crossAxisAlignment: CrossAxisAlignment.center,
children: [
Expanded(
child: Card(
child: Column(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
Container(
width: 130,
child: ListTile(
title: Image.network('https://madvertadvertising.com/media/merza/images/seller.jpeg'),
onTap: (){
// Navigator.push(context, MaterialPageRoute(builder: (context) => ItemsPage()));
},
),
),
Row(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
const SizedBox(width: 8),
],
),
],
),
),
),
Expanded(
child: Card(
child: Column(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
Container(
width: 130,
child: ListTile(
title: Image.network('https://madvertadvertising.com/media/merza/images/delivery.jpeg'),
onTap: (){
Navigator.push(context, MaterialPageRoute(builder: (context) => AwaitingOrdersList(nrc: userData.nrc, paramValueOne: 'order_status', paramValueTwo: 'delivered', barTitle: 'Delivered', isStaff: userData.is_staff.toString(), allowStaffPrivaledges: 'no',)));
},
),
),
const SizedBox(width: 8),
],
),
),
),
],
),
),
Container(
margin: EdgeInsets.symmetric(vertical: 10.0, horizontal: 20.0),
child: Row(
crossAxisAlignment: CrossAxisAlignment.center,
children: [
Expanded(
child: Card(
child: Column(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
Container(
width: 130,
child: ListTile(
title: Image.network('https://madvertadvertising.com/media/merza/images/items.jpeg'),
onTap: (){
Navigator.push(context, MaterialPageRoute(builder: (context) => ItemsPage()));
},
),
),
Row(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
const SizedBox(width: 8),
],
),
],
),
),
),
Expanded(
child: Container(
child: Card(
child: Column(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
Container(
width: 130,
child: ListTile(
title: Image.network('https://madvertadvertising.com/media/merza/images/payment.jpeg'),
onTap: (){
Navigator.push(context, MaterialPageRoute(builder: (context) => DeliverdordersList(nrc: userData.nrc, paramValueOne: 'payment_status', paramValueTwo: 'paid', barTitle: 'Paid orders',)));
}
,
),
),
const SizedBox(width: 8),
],
),
),
),
),
],
),
),
Text('My recent placed orders', style: TextStyle(fontWeight: FontWeight.bold, fontSize: 20)),
Container(
child: OrdersList(),
),
SizedBox(height: 10.0),
],
),
),
drawer: DrawerLogged()
),
);
}else{
return LoadingScreenCustom(page:'home');
}
}
);
}else{
return LoadingScreenCustom(page:'home');
}
}
);
}else{
return LoadingScreenCustom(page:'home');
}
}
);
}
}
}
我不知道在同一页面上添加多个流构建器是否是一个好习惯。
答案 0 :(得分：0)
将 String count_unpaid = snapshot2.data.length.toString(); 行放在 if (snapshot2.hasData) 块内。
将 String count_awaiting = snapshot3.data.length.toString(); 行放在 if (snapshot3.hasData) 块内。