Why my drop down widget occurs an error when include value-CodePudding

Currently I'm trying to implement dropdown using DropdownButton, but there is one problem. In DropdownButton(), if value: is empty, error is not occurred but nothing change when choose. if I assign an arbitrary String variable to value:, the following error will occur.

════════ Exception caught by widgets library ═══════════════════════════════════
The following assertion was thrown building StatefulBuilder(dirty, state: _StatefulBuilderState#65c7c):
Assertion failed:
..\…\material\dropdown.dart:915
items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1
"There should be exactly one item with [DropdownButton]'s value: . \nEither zero or 2 or more [DropdownMenuItem]s were detected with the same value"

My code;

List<String> strings = <String>["Aa", "Bb", "Cc"];
String selectedString = "";

@override
Widget build(BuildContext c) {  ....
showDialog( builder: (context) { 
    return Dialog(builder: (context, setState) {
        return SingleChildScrollView(
          child: Column(   ....

DropdownButton<String>(
                                   value: selectedString,    // IF THIS IS NOT NULL, THE ERROR NOT APPEARD
                                    hint: Text("Choose"),
                                    items: strings.map((String value) {
                                      return DropdownMenuItem<String>(
                                        value: value,
                                        child: Text(value),
                                      );
                                    }).toList(),
                                    onChanged: (value) {
                                      setState(() {
                                        selectedString = value!;
                                      });
                                    },
                                  ),
                                 ........

CodePudding user response：

You have to follow these three steps;

1. List<String> strings = <String>[" ","Aa", "Bb", "Cc"];

2. String selectedString = " ";

3. value: selectedString ,

CodePudding user response：

This was easy to solve.

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

This answer says that selectedString value should be string that exists in strings. Like this selectedString = "Aa";