"There should be exactly one item with [DropdownButton]'s value: Item1" error when using dropdownbutton in flutter

I am trying to use the dropdown menu in my flutter app but getting an error.

Here is the code:

List<String> items = ["Item1", "Item2", "Item3", "Item4"];
String selectedItem = "Item1";
DropdownButton<String>(
  items: items.map(
    (txt) {
      return DropdownMenuItem<String>(
        child: Text(
          "$txt"
        ),
      );
    }
  ).toList(),
  value: selectedItem,
)

In some questions, I saw that we have to initially set a variable to the value present inside our list. I have exactly done that but still getting an error.

Error Message:

There should be exactly one item with [DropdownButton]'s value: Item1. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
'package:flutter/src/material/dropdown.dart':
Failed assertion: line 850 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1'

What is the error here?

Kindly comment if more information is needed.

>Solution :

Here an example, the explanation in the code:

class _MyHomePageState extends State<MyHomePage> {

  List<String> items = ["Item1", "Item2", "Item3", "Item4"];
  String selectedItem = "Item1";

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      body: Column(
        children: [

          Flex(direction: Axis.vertical, children:[
            DropdownButton<String>(
              value: selectedItem,
              onChanged: (_value) {  // update the selectedItem value
                setState(() {
                  selectedItem = _value!;
                });
              },
              items: items
                  .map<DropdownMenuItem<String>>((String _value) => DropdownMenuItem<String>(
                  value: _value, // add this property an pass the _value to it
                  child: Text(_value,)
              )).toList(),
            ),
          ])

        ],
      ),

    );
  }
}