Flutter：带有url的TextFormField抛出NoSuchMethodError：方法“调用”在null上被调用

Question

我正在创建一个带有用于输入服务器URL的字段的登录表单。网址的TextFormField已将keyboardType设置为TextInputType.url。

当我开始在字段中输入网址时，在输入https://

后会引发错误

NoSuchMethodError（NoSuchMethodError：方法“调用”在null上被调用。 接收者：null 尝试调用：call（“ Normal_Mode”，_LinkedHashMap len：0））

此字段引发错误

TextFormField(
  controller: _urlController,
   onChanged: (value) {},
   decoration: const InputDecoration(
   icon: const Icon(Icons.http),
   hintText: 'http://yourdomain.com/',
   labelText: 'Url (no trailing slash /)*',
   ),
   keyboardType: TextInputType.url,
   onSaved: (val) {},
   validator: (value) {
     if (value.isEmpty) {
       return 'Address cannot be empty';
      }
      return null;
  },
)

下面是表单的代码

import 'package:flutter/material.dart';

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

  @override
  State<StatefulWidget> createState() {
    return new LoginFormState();
  }
}

class LoginFormState extends State<LoginForm> {
  final _formKey = new GlobalKey<FormState>();
  final _scaffoldKey = new GlobalKey<ScaffoldState>();
  final _scrollViewKey = new GlobalKey<ScaffoldState>();

  static final TextEditingController _nameController =
      new TextEditingController();
  static final TextEditingController _urlController =
      new TextEditingController();
  static final TextEditingController _userNameController =
      new TextEditingController();
  static final TextEditingController _passwordController =
      new TextEditingController();

  String fcmToken = "";

  bool _checkBoxUrlHasIndex = true;

  @override
  initState() {
    super.initState();
  }

  // Instantiate the Formfields.  Note we provide persisters for each.
  //
  @override
  Widget build(BuildContext context) {
    var loginBtn = Container(
        padding: const EdgeInsets.only(top: 8.0),
        child: new RaisedButton(
          onPressed: () {
            _submit(context);
          },
          child: new Text(
            "LOGIN",
            style: new TextStyle(color: Colors.white),
          ),
          color: Theme.of(context).primaryColor,
        ));

    var loginForm = new Column(
      children: <Widget>[
        Form(
          key: _formKey,
          child:  Column(
            children: <Widget>[
               TextFormField(
                  controller: _nameController,
                  //      onSaved: (val) => _server_name = val,
                  decoration: const InputDecoration(
                      icon: const Icon(Icons.web),
                      hintText: 'Name of site',
                      labelText: 'Server Name *'),
                  //  onSaved: (String value) { person.name = value; },
                  validator: (value) {
                    if (value.isEmpty) {
                      return 'A name is required for this server';
                    }
                    return null;
                  }),
               TextFormField(
                controller: _urlController,
                onChanged: (value) {},
                decoration: const InputDecoration(
                  icon: const Icon(Icons.http),
                  hintText: 'http://yourdomain.com/',
                  labelText: 'Url (no trailing slash /)*',
                ),
                keyboardType: TextInputType.url,
                onSaved: (val) {},
                validator: (value) {
                  if (value.isEmpty) {
                    return 'Address cannot be empty';
                  }
                  return null;
                },
              ),
               Row(
                  mainAxisAlignment: MainAxisAlignment.start,
                  children: <Widget>[
                    Checkbox(
                        value: _checkBoxUrlHasIndex,
                        onChanged: (bool value) {
                          onCheckBoxUrlHasIndexChanged(value);
                        }),
                    Text('Append index.php to address'),
                  ]),
               TextFormField(
                controller: _userNameController,
                decoration: const InputDecoration(
                  icon: const Icon(Icons.person),
                  hintText: 'Username',
                  labelText: 'Username *',
                ),
                validator: (value) {
                  if (value.isEmpty) {
                    return 'Username cannot be empty';
                  }
                  return null;
                },
                //   onSaved: (val) => _username = val,
              ),
               TextFormField(
                controller: _passwordController,
                decoration: const InputDecoration(
                  icon: const Icon(Icons.lock),
                  hintText: 'Password',
                  labelText: 'Password *',
                ),
                obscureText: true,
                validator: (value) {
                  if (value.isEmpty) {
                    return 'Password cannot be empty';
                  }
                  return null;
                },
                //          onSaved: (val) => _password = val,
              ),
              loginBtn,
              Container(
                padding: const EdgeInsets.only(top: 8.0),
                child:  Text('* indicates required field',
                    style: Theme.of(context).textTheme.caption),
              ),
            ],
          ),
        ),
      ],
      crossAxisAlignment: CrossAxisAlignment.center,
    );

    return Scaffold(
      appBar: AppBar(
        title: Text("Login"),
        centerTitle: true,
      ),
      key: _scaffoldKey,
      backgroundColor: Colors.white,
      body: Center(
        child: SingleChildScrollView(
            padding: const EdgeInsets.all(8.0),
            key: _scrollViewKey,
            scrollDirection: Axis.vertical,
            child: Container(
              decoration: BoxDecoration(color: Colors.white),
              child: Container(
                child: loginForm,
                height: 450.0,
                width: 300.0,
              ),
            )),
      ),
    );
  }

  void _submit(BuildContext context) async {
    try {
      final form = _formKey.currentState;
      if (form.validate()) {
        form.save();
      }
    } catch (ex) {}
  }

  void onCheckBoxUrlHasIndexChanged(bool value) {
    setState(() {
      _checkBoxUrlHasIndex = value;
    });
  }
}