Either zero or 2 or more [DropdownMenuItem]s were detected with the same value

Question

I am new to Flutter but I am trying to create a DropdownButtonFormField and it is not working. I am getting an error that says I have duplicate values. What is interesting is that I do not have a list with duplicate values. I have found a similar question on SO and the solution says to initiate the strings with a value and that the user was duplicating a list item, but I have a similar solution for another another list and it seems to work fine. I cannot figure out why it is not working here. Any help is greatly appreciated.

Error Message:

There should be exactly one item with [DropdownButton]'s value: 0. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 1411 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1'
The relevant error-causing widget was: 
  StreamBuilder<UserProfile>

ProfileForm Class:

final List<String> accountType = ['Educator', 'School Administrator', 'Parent'];

  String _currentAccountType;

  @override
  Widget build(BuildContext context) {
    final user = Provider.of<User>(context);
    return StreamBuilder<UserProfile>(
        stream: DatabaseService(uid: user.uid).userProfileData,
        builder: (context, snapshot) {
          if (snapshot.hasData) {
            UserProfile userProfileData = snapshot.data;
            return Form(
              key: _formKey,
              child: Column(
                children: <Widget>[
                  SizedBox(height: 20.0),
                  DropdownButtonFormField(
                    decoration: textInputDecoration,
                    value: _currentAccountType ?? userProfileData.accountType,
                    items: accountType.map((accountType) {
                      return DropdownMenuItem(
                        value: accountType,
                        child: Text(accountType),
                      );
                    }).toList(),
                    onChanged: (val) {
                      setState(() {
                        _currentAccountType = val;
                      });
                    },
                  ),

Database Class

class DatabaseService {
  final String uid;
  DatabaseService({this.uid});

  final CollectionReference userProfileCollection =
      Firestore.instance.collection('user_profile');

  Future updateUserProfile(
      String accountType,
      String birthDate,
      String courseName,
      String dateJoined,
      String email,
      String firstName,
      String lastName,
      String schoolName,
      String title) async {
    return await userProfileCollection.document(uid).setData({
      'accountType': accountType,
      'birthDate': birthDate,
      'courseName': courseName,
      'dateJoined': dateJoined,
      'email': email,
      'firstName': firstName,
      'lastName': lastName,
      'schoolName': schoolName,
      'title': title,
    });
  }

  //User Profile from snapshot
  List<Profile> _userProfileListFromSnapshot(QuerySnapshot snapshot) {
    return snapshot.documents.map((doc) {
      return Profile(
        accountType: doc.data['accountType'] ?? '',
        birthDate: doc.data['birthDate'] ?? '',
        courseName: doc.data['courseName'] ?? '',
        dateJoined: doc.data['dateJoined'] ?? '',
        email: doc.data['email'] ?? '',
        firstName: doc.data['firstName'] ?? '',
        lastName: doc.data['lastName'] ?? '',
        schoolName: doc.data['schoolName'] ?? '',
        title: doc.data['title'] ?? '',
      );
    }).toList();
  }

  UserProfile _userProfileFromSnapshot(DocumentSnapshot snapshot) {
    return UserProfile(
      uid: uid,
      accountType: snapshot.data['accountType'],
      birthDate: snapshot.data['birthDate'],
      courseName: snapshot.data['courseName'],
      dateJoined: snapshot.data['dateJoined'],
      email: snapshot.data['email'],
      firstName: snapshot.data['firstName'],
      lastName: snapshot.data['lastName'],
      schoolName: snapshot.data['schoolName'],
      title: snapshot.data['title'],
    );
  }

  Stream<List<Profile>> get userProfile {
    return userProfileCollection.snapshots().map(_userProfileListFromSnapshot);
  }

  Stream<UserProfile> get userProfileData {
    return userProfileCollection
        .document(uid)
        .snapshots()
        .map(_userProfileFromSnapshot);
  }
}

Answer 1

userProfileData.accountType is '0', not 'Educator' or 'School Administrator' or 'Parent'.

Success: value must in items.value

 final List<String> accountType = ['Educator', 'School Administrator', 'Parent'];

 DropdownButtonFormField(
                    decoration: textInputDecoration,
                    value: accountType[0],
                    items: accountType.map((accountType) {
                      return DropdownMenuItem(
                        value: accountType,
                        child: Text(accountType),
                      );
                    }).toList(),
                    onChanged: (val) {
                      setState(() {
                        _currentAccountType = val;
                      });
                    },
                  ),

Failed: There should be exactly one item with [DropdownButton]'s value: hahaha

 final List<String> accountType = ['Educator', 'School Administrator', 'Parent'];

 DropdownButtonFormField(
                    decoration: textInputDecoration,
                    value: 'hahaha',
                    items: accountType.map((accountType) {
                      return DropdownMenuItem(
                        value: accountType,
                        child: Text(accountType),
                      );
                    }).toList(),
                    onChanged: (val) {
                      setState(() {
                        _currentAccountType = val;
                      });
                    },
                  ),

Answer 2

It seems that there is some clash when using 'hint:' and 'value:' simultaneously. My approach was to add a global _selected in the Widget's State:

bool _selected;

and then in the DropdownButton itself:

value: _selected ? _userChoice: null,

In this way you actually use one of the item values once they are set.

Here is an entire example taken from some traning code (excuse the silliness ;)

class FavoriteCity extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _FavoriteCityState();
  }
}

class _FavoriteCityState extends State<FavoriteCity> {
  String nameCity = '';
  String _loveLevel = '';
  bool _selected = false;
  var _howMuchLoved = ['A little', 'So so', 'Quite a bit', 'A lot', 'Greatly'];

  @override
  Widget build(BuildContext context) {
    print('Widget Built');
    return Scaffold(
      appBar: AppBar(
        title: Text('Favorite city app'),
        elevation: 8.0,
      ),
      body: Container(
        margin: EdgeInsets.all(20.0),
        child: Column(
          children: <Widget>[
            TextField(
              onSubmitted: (String userInput) {
                setState(() {
                  print('State rebuilt');
                  nameCity = userInput;
                });
              },
            ),
            DropdownButton<String>(
              hint: Text('How much do you love the city?'),
              items: _howMuchLoved.map((String myMenuItem) {
                return DropdownMenuItem<String>(
                  value: myMenuItem,
                  child: Text(myMenuItem),
                );
              }).toList(),
              onChanged: (String valueSelectedByUser) {
                _dropDownItemSelected(valueSelectedByUser);
              },
              value: _selected ? _loveLevel : null,
              isDense: true,
            ),
            Padding(
              padding: EdgeInsets.all(20.0),
              child: Text(
                'Your favorite city is $nameCity ! \n ... and you love it $_loveLevel',
                style: TextStyle(
                  fontSize: 20.0,
                  fontStyle: FontStyle.italic,
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }

  void _dropDownItemSelected(String valueSelectedByUser) {
    setState(() {
      this._loveLevel = valueSelectedByUser;
      _selected = true;
    });
  }

Answer 3

In my case, below change resolved the error:

Error scenario:

var itemList=['Alpha','Beta','Cat'];
var itemSelected='Zebra';

Working scenario:

var itemList=['Alpha','Beta','Cat'];
var itemSelected='Cat'; //Can be Alpha, Beta or Cat but not any other value

Dropdown widget code:

DropdownButton<String>(
                    items: itemList.map((String singleItem){
                          return DropdownMenuItem<String>(
                            value: singleItem,
                            child:Text(singleItem)
                          );
                    }).toList(),

                    onChanged: (String itemChosen){
                      setState(() {
                        this.itemSelected=itemChosen;
                      });
                    },
                   value:itemSelected ,

                  ),

Having the 'itemSelected' variable value same as one of the elements in the list resolved the problem for me.