Updating Dropdown Data In Flutter Gives Error

I am in the process of developing a widget that consists of a dropdownbutton in Flutter. This widget creates dropdownmenuitems after making a request to a url the user has passed in.

I have seen the call being made and the response being valid, but when I use setState to update the data source for the dropdown I get the following error:

'package:flutter/src/material/dropdown.dart': Failed assertion: line 560 pos 15: 'items == null || I/flutter (11514): items.isEmpty || value == null || items.where((DropdownMenuItem item) => item.value == I/flutter (11514): value).length == 1': is not true.

I tried researching this error and found no helpful answers. The code is below:

class _MyWidgetState extends State<MyWidget> {

List<DropdownMenuItem<String>> _data = [];
String _selected = '';

@override
void initState() {
   super.initState();
   _loadData();
 }
 void _loadData() async {
    if (widget.urlToFetchData.isNotEmpty) {
      var response = await http.get(widget.urlToFetchData);
      if (response.statusCode == 200) {
         Map<String, dynamic> jsonResponse = convert.jsonDecode(response.body);
         jsonResponse.forEach((key, value){
           setState(() {
                  this._data.add(new DropdownMenuItem(
                                  child: new Text(value.toString()),
                                  value: value.toString(),
                             ));
           });
         });
      } else {
         print("Request failed with status: ${response.statusCode}.");
       }
    }
  }
}

@override
Widget build(BuildContext context) {
   if (_data.length == 0) {
      return new Container();
   } else {
        return Column(
            children: <Widget> [
                  new Text(
                    widget.dropdownLabelTitle
                 ),
                DropdownButton(
                     value: _selected,
                     items: _data,
                     hint: new Text(widget.defaultOptionText),
                     onChanged: (value) {
                        _selected = value;
                        widget.valueReturned(_selected);
                        setState(() {

                        });
                     }
                   )
                ],
              );
            }
       }
    }

Now, I am fully aware that at the initialization of the widget, the items field in the dropdownbutton is initialized with an empty list, but I thought that by calling setState when the http call finishes, it will update the value.

I have tried different methods of updating the data(by creating a local list and then using addAll or just by assignment), but I get the same error.

Anyone know how to solve this issue?

You are getting this error because dropdown menu is trying to select item that does not exists.

DropdownButton(
        value: _selected, # < here you are trying to select `String _selected = '';`
        items: _data,

To solve this simple remove value or set it somewhere.