Flutter TextFormField: The keyboard appeared and instantly disappeared

When I click the TextFormField, the keyboard appears, then it disappears instantly, just like blinking. I wonder that whether it is because the lacking of some parameters? Or some other problems? Thanks. Here are my relative codes.

final _formKey = GlobalKey<FormState>();
String? _account;
Widget _showAccountInput() {
    return Padding(
      padding: const EdgeInsets.fromLTRB(15.0, 10.0, 0.0, 0.0),
      child: new TextFormField(
        maxLines: 1,
        obscureText: true,
        autofocus: false,
        style: TextStyle(fontSize: 15),
        decoration: new InputDecoration(
            border: InputBorder.none,
            hintText: 'input',
            icon: new Icon(
              Icons.lock,
              color: Colors.grey,
            )),
        onSaved: (value) => _account = value?.trim(),
      ),
    );
  }
  Widget build(BuildContext context) {
    // TODO: implement build
    return Scaffold(
      appBar: new AppBar(
        title: new Text("Second Screen"),
      ),
      body: Form(
        key: _formKey,
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            _showAccountInput()


          ],
        ),
      ),
    );
  }
}

if your build method is in a StatelessWidget, try to change it to a StatefulWidget