Here is my code

  class FakturPage extends StatefulWidget {
  const FakturPage({Key? key}) : super(key: key);

  static String id = 'fakturpage';

  @override
  State<FakturPage> createState() => _FakturPageState();
}

class _FakturPageState extends State<FakturPage> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          DropdownTextfield(),
        ],
      ),
    );
  }
}

and this is the DropdownTextfield widget code, where I tried to make a UI that shows a Textfield and a DropdownButton that has options inputted from the Textfield above it.

class DropdownTextfield extends StatefulWidget {

  @override
  _DropdownTextfieldState createState() => _DropdownTextfieldState();
}

class _DropdownTextfieldState extends State<DropdownTextfield> {

  var selectedOption;
  TextEditingController textfieldValue = TextEditingController();
  final List<String> options = [];

  @override
  void initState() {
    super.initState();
    selectedOption = (options.isNotEmpty ? options[0] : null)!;
  }

  @override
  Widget build(BuildContext context) {
    return Column(
      children: <Widget>[
        TextField(
          onChanged: (value) {
            setState(() {
              value = textfieldValue.text;
            });
          },
        ),
        DropdownButton<String>(
          value: selectedOption,
          onChanged: (value) {
            setState(() {
              selectedOption = value!;
            });
          },
          items: options.map((option) {
            return DropdownMenuItem<String>(
              value: option,
              child: Text(option),
            );
          }).toList(),
        ),
        TextButton(
          onPressed: () {
            setState(() {
              options.add(textfieldValue.text);
            });
          },
          child: Text("Add Option"),
        ),
      ],
    );
  }
}

I don’t know what went wrong. I tried to figure out if my custom dropdown widget is the problem but I can’t find anything wrong. I’ve been trying to fix this for almost an hour without finding the solution, since I’m new to the mobile development world. Can you help me?

>Solution :

The issue is you are using null-assert(!) though you are doing null check.

Replace

selectedOption = (options.isNotEmpty ? options[0] : null)!;

With

selectedOption = options.isNotEmpty ? options[0] : null;