Flutter DropdownButton is Failed - dart

Question

I have created two DropdownButtons in the appBar and in the Body of a Scaffold.The Top DropdownButton changes the language whenever it is selected.After I selected One Conversion rate from the second Dropdown Button and Change the Language, the code crushes.Here is the Code..
import 'package:flutter/material.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return new MaterialApp(
title: 'Flutter Demo',
theme: new ThemeData(
primarySwatch: Colors.blue),
home: new MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => new _MyHomePageState();
}
class User {
User(this.name);
final String name;
}
class CurrencyClass {
final String currencyType;
CurrencyClass(this.currencyType);
}
class _MyHomePageState extends State<MyHomePage> {
User selectedUser;
CurrencyClass selectedCurrency;
List<User> users = <User>
[User('English'),User('Amharic'),User('Oromigna'),
User('Tigrigna'),User('Somali')];
List<CurrencyClass> currency = <CurrencyClass>[
CurrencyClass('USD'),
CurrencyClass('GBP'),
CurrencyClass('CAD'),
CurrencyClass('AUD'),
CurrencyClass('EURO'),CurrencyClass('SAR'),];
#override
void initState() {
super.initState();
selectedUser = users[0];
}
#override
Widget build(BuildContext context) {
return new Scaffold(
appBar: new AppBar(
title: new Text(widget.title),
actions: <Widget>[
DropdownButton<User>(
value: selectedUser,
onChanged:(User newValue){
setState(() {
selectedUser = newValue;
if(newValue == users[0]){
currency = <CurrencyClass>[CurrencyClass('English'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
}
else if(newValue == users[1]){
currency = <CurrencyClass>[CurrencyClass('Amharic'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
}
else if(newValue == users[2]){
currency = <CurrencyClass>[CurrencyClass('Oromigna'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
} else if(newValue == users[3]){
currency = <CurrencyClass>[CurrencyClass('Tigringa'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
} else {
currency = <CurrencyClass>[CurrencyClass('Somaligna'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
}
});
},
items: users.map((User user){
return new DropdownMenuItem<User>(
value: user,
child:Text(
user.name,
style: TextStyle(color: Colors.black),
),
);
}).toList(),
)
],
),
body: new Center(
child: new Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
new Padding(padding: EdgeInsets.symmetric(vertical: 50.0),),
new Row(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Text('Amount',style: TextStyle(fontSize: 20.0),),
Padding(padding: EdgeInsets.symmetric(horizontal: 15.0)),
Text('Birr',style: TextStyle(fontSize: 24.0),),
Padding(padding: EdgeInsets.symmetric(horizontal: 15.0)),
DropdownButton<CurrencyClass>(
value: selectedCurrency,
onChanged: (CurrencyClass newExchange){
setState(() {
selectedCurrency = newExchange;
});
},
items: currency.map((CurrencyClass user2){
return DropdownMenuItem<CurrencyClass>(
value: user2,
child: Text(
user2.currencyType,
style: TextStyle(color: Colors.red),
),
);
}).toList(),
),
],
),
],
),
),
);
}
}
Here is the Error when I select the top DropdownButton after selecting some value in the bottom dropdownButton
Launching lib/main.dart on XT1068 in debug mode...
Built build/app/outputs/apk/debug/app-debug.apk.
I/FlutterActivityDelegate(11200): onResume setting current activity to
this
I/flutter (11200): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY
╞═══════════════════════════════════════════════════════════
I/flutter (11200): The following assertion was thrown building
MyHomePage(dirty, state: _MyHomePageState#3ce11):
I/flutter (11200): 'package:flutter/src/material/dropdown.dart': F
Failed assertion: line 481 pos 15: 'value == null ||
I/flutter (11200): items.where((DropdownMenuItem<T> item) =>
item.value == value).length == 1': is not true.
I/flutter (11200): Either the assertion indicates an error in the
framework itself, or we should provide substantially
I/flutter (11200): more information in this error message to help you
determine and fix the underlying cause.

Answer 1

The problem in the original post is when setting the currency values on selecting the bar dropdown. You need to also set the selectedCurrency.
selectedCurrency = currency[0];
Or do what Gunter did in the edit. Just do not change the currency dropdown so there is no problem at all.
onChanged:(User newValue){
setState(() {
selectedUser = newValue;
if(newValue == users[0]){
currency = <CurrencyClass>[CurrencyClass('English'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
}
else if(newValue == users[1]){
currency = <CurrencyClass>[CurrencyClass('Amharic'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
}
else if(newValue == users[2]){
currency = <CurrencyClass>[CurrencyClass('Oromigna'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
}
else if(newValue == users[3]){
currency = <CurrencyClass>[CurrencyClass('Tigringa'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
}
else {
currency = <CurrencyClass>[CurrencyClass('Somaligna'),CurrencyClass('CAD'),CurrencyClass('AUD'),CurrencyClass('EURO'),CurrencyClass('SAR')];
}
selectedCurrency = currency[0];
});
},