它不允许我更改表单内的单选按钮，无论选择什么，值也不会更改。我尝试过表单外的代码，它工作正常，但当我把它放在表单内，然后它停止工作，原因我不知道。

FloatingActionButton(
        onPressed: () {
          showDialog(
              context: context,
              builder: (BuildContext context) {
                return AlertDialog(
                  title: Text('New User'),
                  content: Stack(
                    children: <Widget>[
                      Positioned(
                        right: -40.0,
                        top: -40.0,
                        child: InkResponse(
                          onTap: () {
                            Navigator.of(context).pop();
                          },
                          child: CircleAvatar(
                            child: Icon(Icons.close),
                            backgroundColor: Colors.blue,
                          ),
                        ),
                      ),
                      Form(
                        child: SingleChildScrollView(
                          child: Column(
                            mainAxisSize: MainAxisSize.min,
                            crossAxisAlignment: CrossAxisAlignment.stretch,
                            children: <Widget>[
                              Padding(
                                padding: EdgeInsets.all(8.0),
                                child: TextFormField(
                                  controller: nameController,
                                  decoration: InputDecoration(
                                    labelText: 'Name',
                                  ),
                                ),
                              ),
                              Padding(
                                padding: EdgeInsets.all(8.0),
                                child: TextFormField(
                                  controller: surnameController,
                                  decoration: InputDecoration(
                                    labelText: 'Surname',
                                  ),
                                ),
                              ),
                              Padding(
                                padding: EdgeInsets.all(8.0),
                                child: TextFormField(
                                  controller: userController,
                                  decoration: InputDecoration(
                                    labelText: 'Username',
                                  ),
                                ),
                              ),
                              Padding(
                                padding: EdgeInsets.all(8.0),
                                child: TextFormField(
                                  controller: passController,
                                  decoration: InputDecoration(
                                    labelText: 'Password',
                                  ),
                                ),
                              ),
                              Padding(
                                padding: EdgeInsets.all(8.0),
                                child: TextFormField(
                                  controller: mobileController,
                                  decoration: InputDecoration(
                                    labelText: 'Mobile num',
                                  ),
                                ),
                              ),
                              Padding(
                                padding: EdgeInsets.all(8.0),
                                child: TextFormField(
                                  controller: emailController,
                                  decoration: InputDecoration(
                                    labelText: 'Email',
                                  ),
                                ),
                              ),
                              //radioButtons
                              Container(
                                padding: EdgeInsets.all(20),
                                child: Column(
                                  children: [
                                    Text(
                                      "What is your gender?",
                                      style: TextStyle(fontSize: 18),
                                    ),
                                    Divider(),
                                    RadioListTile(
                                      title: Text("Male"),
                                      value: "male",
                                      groupValue: gender,
                                      onChanged: (value) {
                                        setState(() {
                                          gender = value.toString();
                                        });
                                      },
                                    ),
                                    RadioListTile(
                                      title: Text("Female"),
                                      value: "female",
                                      groupValue: gender,
                                      onChanged: (value) {
                                        setState(() {
                                          gender = value.toString();
                                        });
                                      },
                                    ),
                                    RadioListTile(
                                      title: Text("Other"),
                                      value: "other",
                                      groupValue: gender,
                                      onChanged: (value) {
                                        setState(() {
                                          gender = value.toString();
                                        });
                                      },
                                    )
                                  ],
                                ),
                              ),
                              Padding(
                                padding: const EdgeInsets.all(8.0),
                                child: FloatingActionButton(
                                  child: Text("Submit"),
                                  onPressed: () {
                                    //createUser();
                                    print(gender);

                                    Navigator.pop(context);
                                  },
                                ),
                              )
                            ],
                          ),
                        ),
                      ),
                    ],
                  ),
                );
              });
        },
        child: Icon(Icons.person),
        backgroundColor: Colors.blue,
      ),

它有几个文本字段，然后是单选按钮，gender变量被声明为字符串。