DropdownButtonFormField Empty State Issue

dropdown_button_form_field_issue.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'DropdownButtonFormField',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      routes: {
        '/': (context) => HomePage(),
      },
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  HomePageState createState() {
    return new HomePageState();
  }
}

class HomePageState extends State<HomePage> {
  String _selected;
  List<String> items;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Home'),
      ),
      body: Column(
        children: <Widget>[
          DropdownButtonFormField<String>(
            items: items == null
                ? []
                : items
                    .map(
                      (String item) => DropdownMenuItem<String>(
                            child: Text(item),
                            value: item,
                          ),
                    )
                    .toList(),
            value: _selected,
            onChanged: items == null
                ? null
                : (String value) {
                    setState(() {
                      _selected = value;
                    });
                  },
          ),
        ],
      ),
    );
  }
}