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

I am trying to create a dropdown button in Flutter. I am getting a List from my database then I pass the list to my dropdownButton everything works the data is shown as intended but when I choose an element from it I get this error:

There should be exactly one item with [DropdownButton]'s value: Instance of 'Tag'. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 805 pos 15: 'items == null || items.isEmpty || value == null ||
          items.where((DropdownMenuItem<T> item) {
            return item.value == value;
          }).length == 1'

I tried setting DropdownButton value to null it works but then I can't see the chosen element.

Here is my code:

FutureBuilder<List<Tag>>(
    future: _tagDatabaseHelper.getTagList(),
    builder: (BuildContext context, AsyncSnapshot<List<Tag>> snapshot) {
      if (!snapshot.hasData) {
        return Center(
          child: CircularProgressIndicator(),
        );
      }
      return ListView(
        children: <Widget>[
          SizedBox(
            height: MediaQuery.of(context).size.height * 0.2,
          ),
          Container(
            margin: EdgeInsets.symmetric(
                horizontal: MediaQuery.of(context).size.width * 0.07),
            child: Theme(
              data: ThemeData(canvasColor: Color(0xFF525A71)),
              child: DropdownButton<Tag>(
                value: _selectedTag,
                isExpanded: true,
                icon: Icon(
                  Icons.arrow_drop_down,
                  size: 24,
                ),
                hint: Text(
                  "Select tags",
                  style: TextStyle(color: Color(0xFF9F9F9F)),
                ),
                onChanged: (value) {
                  setState(() {
                    _selectedTag = value;
                  });
                },
                items: snapshot.data.map((Tag tag) {
                  return DropdownMenuItem<Tag>(
                    value: tag,
                    child: Text(
                      tag.tagTitle,
                      style: TextStyle(color: Colors.white),
                    ),
                  );
                }).toList(),
                value: _selectedTag,
              ),
            ),
          ),

I used futureBuilder to get my List from database.

Well, since no problem has an exact same solution. I was facing the same issue with my code. Here is How I fixed this.

CODE of my DropdownButton:

DropdownButton(
   items: _salutations
         .map((String item) =>
             DropdownMenuItem<String>(child: Text(item), value: item))
         .toList(),
    onChanged: (String value) {
       setState(() {
         print("previous ${this._salutation}");
         print("selected $value");
         this._salutation = value;
            });
          },
     value: _salutation,
),

The Error

In the code snippet below, I am setting the state for a selection value, which is of type String. Now problem with my code was the default initialization of this selection value. Initially, I was initializing the variable _salutation as:

String _salutation = ""; //Notice the empty String.

This was a mistake!

Initial selection should not be null or empty as the error message correctly mentioned.

'items == null || items.isEmpty || value == null ||

And hence the crash:

Solution
Initialize the value object with some default value. Please note that the value should be the one of the values contained by your collection. If it is not, then expect a crash.

  String _salutation = "Mr."; //This is the selection value. It is also present in my array.
  final _salutations = ["Mr.", "Mrs.", "Master", "Mistress"];//This is the array for dropdown

Might also get this error if trying to set value of dropdown with a class instance;

  var tag1 = Tag();
  var tag2 = Tag();
  print(tag1 == tag2); // prints false, dropwdown computes that value is not present among dropdown options

To solve this override operator ==:

class Tag{
 String name = "tag";

  @override
  bool operator ==(Object other) => other is Tag && other.name == name;

  @override
  int get hashCode => name.hashCode;
}

or use https://pub.dev/packages/equatable lib

class Tag extends Equatable{
 String name = "tag";

  @override
  List<Object> get props => [name];
}

I had the same problem. The solution is simple: you have to be sure that the String that is your default dropdownvalue is contained in the list that you want to use in your dropdownmenu. If you wanted to, let’s say, use a list from an api, you should be sure to know at least one value of that list, so that you could assign it to the variable that is your default dropdownvalue.

Here I want display a list that I obtain from an api. In order to not obtain the error, I set my defaultdropdownvalue with the name ‘Encajes’ that is one of the existing categories that my list contains.

String dropdownValue = "Encajes";

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

Code of my dropdown

child: DropdownButton(
      items: _currencies.map((String value) {
        return DropdownMenuItem<String>(
          child: Text(value),
          value: value,
        );
      }).toList(),
      value: 'Rupees',
      onChanged: (String newValueSelected) {
        // Your code to execute, when a menu item is selected from 
        dropdown
      },
))
var _currencies = ['Rupee','Dollar','Pound'];

I faced same error because the value in the dropdown code block is not matching with any of the fields in _currencies

just make the tag class extend from Equatable and pass the attributes to the props.. this did the trick for me.

class Tag extends Equatable{
  String id;
  String name;

  Tag(this.id, this.name);

  @override
  List<Object> get props => [id,name];

}