尝试调用：[]("number")

Question

我正在尝试从实时数据库中获取一个值并将其与用户提供的输入相匹配。如果匹配，则导航到下一页，否则抛出错误，但在我正在实现从实时获取价值的方法的部分，我收到以下错误：

E/flutter (17592): [ERROR:flutter/lib/ui/ui_dart_state.cc(186)] 未处理的异常: NoSuchMethodError: 方法 '[]' 在 null 上调用。 E/颤振（17592）：接收器：空 E/flutter (17592)：尝试调用：

import 'package:country_code_picker/country_code_picker.dart';
import 'package:firebase_database/firebase_database.dart';
import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'OTPController.dart';

class LoginScreen extends StatefulWidget {
  const LoginScreen({Key key}) : super(key: key);

  @override
  _LoginScreenState createState() => _LoginScreenState();
}

class _LoginScreenState extends State<LoginScreen> {
  DataSnapshot snapshot;

  Query _ref;
  String dialCodeDigits = "+977";
  TextEditingController _controller = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SingleChildScrollView(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            SizedBox(height: 100),
            Padding(
              padding: const EdgeInsets.only(left: 28.0, right: 28.0),
              child: Image.asset("assets/prologo.png"),
            ),
            Container(
              margin: EdgeInsets.only(top: 10),
              child: Center(
                  child: Text(
                "Phone (OTP) Authentication",
                style: TextStyle(fontWeight: FontWeight.bold, fontSize: 20),
              )),
            ),
            SizedBox(
              height: 50,
            ),
            SizedBox(
              height: 60,
              width: 400,
              child: CountryCodePicker(
                onChanged: (country) {
                  setState(() {
                    dialCodeDigits = country.dialCode;
                  });
                },
                initialSelection: "NPL",
                showCountryOnly: false,
                showOnlyCountryWhenClosed: false,
                favorite: ["+91", "IND", "+1", "USA"],
              ),
            ),
            Container(
                margin: EdgeInsets.only(top: 10, right: 10, left: 10),
                child: TextField(
                  decoration: InputDecoration(
                      hintText: "Phone Number",
                      prefix: Padding(
                        padding: EdgeInsets.all(4),
                        child: Text(dialCodeDigits),
                      )),
                  maxLength: 12,
                  keyboardType: TextInputType.number,
                  controller: _controller,
                )),
            Container(
              margin: EdgeInsets.all(15),
              width: double.infinity,
              child: ElevatedButton(
                onPressed: () async {
                  var numberPhone = dialCodeDigits + _controller.text;

                  FirebaseDatabase.instance
                      .reference()
                      .child('Subscribers')
                      .child('number')
                      .equalTo(numberPhone)
                      .once()
                      .then((DataSnapshot snapshot) {
                    Map numPhone = snapshot.value;
                    var phoneNumber = numPhone['number'];
                    print(phoneNumber);
                  });

                  if ('+917340868077' == numberPhone) {
                    Navigator.of(context).push(MaterialPageRoute(
                        builder: (c) => OTPController(
                            phoneNumber: _controller.text,
                            codeDigit: dialCodeDigits)));
                  }
                },
                child: Text(
                  'Next',
                  style: TextStyle(
                      color: Colors.white, fontWeight: FontWeight.bold),
                ),
              ),
            )
          ],
        ),
      ),
    );
  }
}

Answer 1

您的错误很可能出现在这一行：
var phoneNumber = numPhone['number'];

由于numPhone 可以为空，您可以像这样进行检查：

//... other lines
.then((DataSnapshot snapshot) {
  if (snapshot.value == null) {
    // do sth with the null return
    print('no data');
    return;
  }
  Map numPhone = snapshot.value;
  var phoneNumber = numPhone['number'];
  print(phoneNumber);
});