DropdownButton error after selection (HTTP-JSON-FutureBuilder) - dart

Question

I have some FutureBuilder that returns a DrobdownButton filled with the result of a HTTP request, using JSON response. I get the DropdownButton filled and everything seems to work just fine. The problem is after selecting an element I got an error:
'package:flutter/src/material/dropdown.dart': Failed assertion: line 609 pos 15: 'items == null ||
I/flutter ( 5210): items.isEmpty || value == null || items.where((DropdownMenuItem item) => item.value ==
I/flutter ( 5210): value).length == 1': is not true.
I understand that, for some reason, the DropdownButton get empty after selection or so...
I tried this code:
https://inducesmile.com/google-flutter/how-to-populate-dropdownbutton-using-json-api-in-flutter/
But in this case, after selection, the selected value goes to another widget. The thing I want is to use de value: parameter of the DropdownButton.
In the code above, the value: parameter is commented. I tried this code with no comment on that line and remove the SizedBox. The only thing I want is that the value is selected on the same DropdownButton
import 'dart:async';
import 'dart:io';
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
Items _currentItem;
Future<List<Items>> _getItemsData() async {
List<Items> dataItems = new List<Items>();
for (int i = 1; i <= 3; i++) {
dataItems.add(
Items.fromJson(
{'id':i,'name':'Elem ${i}'}
)
);
}
sleep(new Duration(seconds: 5));
return dataItems;
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text(widget.title),
),
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
new FutureBuilder(
future: _getItemsData(),
builder: (BuildContext context, AsyncSnapshot<List<Items>> snapshot) {
if (!snapshot.hasData) {
return CircularProgressIndicator();
} else {
return new Container(
width: 280,
child: new DropdownButton<Items>(
items: snapshot.data.map((items) => DropdownMenuItem<Items>(
child: Text(items.name),
value: items,
)).toList(),
onChanged: (Items value) {
setState(() {
_currentItem = value;
});
},
isExpanded: true,
value: _currentItem,
hint: Text('Select one...')
)
);
}
}
)
]
)
)
);
}
}
class Items {
int id;
String name;
Items({this.id, this.name});
factory Items.fromJson(Map<String, dynamic> json) {
return Items (
id: json['id'],
name: json['name']
);
}
}