Why DropDownButton doesn't work in Flutter?

I want to make a simple DropDownButton in Flutter. And I don't understand the error I'm getting:

And here is the code of the page with the error:

class MyPage extends StatefulWidget {
  MyPage({super.key});

  @override
  State<MyPage> createState() => _Example();
}

class _Example extends State<MyPage> {
  List<String> items = ['small', 'medium', 'large', 'extra-large'];
  String? selectedItem = 'samll';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: const Text('HDBOOKS'),
        ),
        body: DropdownButton(
          value: selectedItem,
          items: items
              .map((e) => DropdownMenuItem(value: e, child: Text(e)))
              .toList(),
          onChanged: (item) => setState(() {
            selectedItem = item!;
          }),
        ));
  }
}

You have a typo in line String? selectedItem = **'samll'**; that is not matching with the available options. Below is the code working now with some improvements

import "package:flutter/material.dart";

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyPage(),
    );
  }
}

class MyPage extends StatefulWidget {
  MyPage({super.key});

  @override
  State<MyPage> createState() => _Example();
}

class _Example extends State<MyPage> {
  List<String> items = ['small', 'medium', 'large', 'extra-large'];
  String? selectedItem = 'small';

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: const Text('HDBOOKS'),
        ),
        body: DropdownButton<String>(
          value: selectedItem,
          items: items.map<DropdownMenuItem<String>>((String value) {
            return DropdownMenuItem<String>(
              value: value,
              child: Text(
                value,
                style: TextStyle(fontSize: 30),
              ),
            );
          }).toList(),
          onChanged: (String? newValue) => setState(() {
            selectedItem = newValue!;
          }),
        ));
  }
}