flutter: There should be exactly one item with [DropdownButton]'s value: A. Either zero or 2 or more [DropdownMenuItem]s were detected with the same

Question

I made DropdownButton with Flutter, and I've got the error "There should be exactly one item with [DropdownButton]'s value: A. Either zero or 2 or more [DropdownMenuItem]s were detected with the same value 'package:flutter/src/material/dropdown.dart': Failed assertion: line 890 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem item) { return item.value == value; }).length == 1'"
here is the code

class HomeController extends GetxController {
 List<String> sections = [
    'A',' B',' C', ' D',' E',' F',' G'];
  String selectedLetter = "A";

  void setSectionLetter(String s) {
    selectedLetter = s;
    update();
  }
}
GetBuilder<HomeController>(builder: (contH) {
              return DropdownButton<String?>(
                items: contH.sections
                    .map((e) => DropdownMenuItem<String?>(
                        child: HDW().title(context: context, text: e)))
                    .toList(),
                value: contH.selectedLetter.isNotEmpty
                    ? contH.selectedLetter
                    : null,
                onChanged: (value) {
                  contH.setSectionLetter(value!);
                },
                isExpanded: true,
                icon: const Icon(Icons.keyboard_arrow_down),
              );
            })

Answer 1

If contH.selectedLetter.isNotEmpty is false , the value is set to null . However, sections does not contain the option null . This explains the error: zero [DropdownMenuItem]s were detected with the value null . Either add null to the sections or change null to a value that is already in sections .

EDIT: I tried to make a DartPad sample for this and found that the actual problem is that you're not passing a value to your DropdownMenuItems. Aside from a child that is used to represent a value, there is also a value parameter that stores the actual value of that option. See line 44 in this DartPad: https://dartpad.dev/?id=57eb35e737ca1420aca6a833b8bb716f

You can use the exact same line in your code I would say.