title: Flutter TextFormField moving up when TextHelp is visible
body: <p>I don't understood what's happening, but when the form got erros the helper text message is moving the TextFormField. I tried increasing the height but I could not fix. </p>
  
  <p>Anyone knows what's happening?</p>
  
  <p>Look the image:</p>
  
  <p><a href="https://i.stack.imgur.com/3gUmq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3gUmq.png" alt="enter image description here"></a></p>
  
  <p>Follow the code:</p>
  
  <pre><code>Row(
                        crossAxisAlignment: CrossAxisAlignment.center,
                        mainAxisAlignment: MainAxisAlignment.center,
                        children: [
                          Container(
                              alignment: Alignment.centerRight,
                              child: CountryCodePicker(
                                showFlagMain: true,
                                onChanged: print,
                                initialSelection:
                                    I18n.of(context).locale.countryCode,
                                favorite: ['+55', 'BR'],
                                showCountryOnly: false,
                                showOnlyCountryWhenClosed: false,
                                textStyle: TextStyle(
                                    color: Colors.white, fontSize: 19.0),
                                flagWidth: 40.0,
                              )),
                          Container(
                              width: 200,
                              alignment: Alignment.center,
                              child: TextFormField(
                                keyboardType: TextInputType.phone,
                                controller: _phoneTextController,
                                inputFormatters: [
                                  MaskTextInputFormatter(
                                      mask: "(##) #####-####",
                                      filter: {"#": RegExp(r'[0-9]')})
                                ],
                                autocorrect: false,
                                autofocus: false,
                                style: TextStyle(
                                    color: Colors.white, fontSize: 19.3),
                                cursorColor: Colors.yellow,
                                decoration: const InputDecoration(
                                    border: InputBorder.none,
                                    hintText: '(99) 99999-9999',
                                    filled: true,
                                    hintStyle: TextStyle(color: Colors.grey)),
                                validator: (String value) =&gt;
                                    phoneValidator(value),
                              ))
                        ],
                      ),
                      SizedBox(height: 20),
                      RaisedButton(
                        child: Text('Send'),
                        onPressed: () {
                          if (this._form.currentState.validate()) {
                            print(this._unmask(this._phoneTextController.text));
                          }
                        },
                      )
                    ],
  </code></pre>
  
  <p>Thanks.</p>