آموزش طراحی منو آبشاری در فلاتر با ویجت DropDownButton

import "package:flutter/material.dart";
import 'package:flutter_widgets/const/_const.dart';

class DropDownButtonPage extends StatefulWidget {
  @override
  _DropDownButtonState createState() => _DropDownButtonState();
}

class _DropDownButtonState extends State<DropDownButtonPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(PageName.DROP_DOWN_BUTTON),
      ),
      body: SingleChildScrollView(
        child: Column(
          children: <Widget>[
            //our code.
            SizedBox(height: 600)
          ],
        ),
      ),
    );
  }
}

 DropdownButton _normalDown() => DropdownButton<String>(
        items: [
          DropdownMenuItem(
            value: "1",
            child: Text(
              "First",
            ),
          ),
          DropdownMenuItem(
            value: "2",
            child: Text(
              "Second",
            ),
          ),
        ],
        onChanged: (value) {
          setState(() {
            _value = value;
          });
        },
        value: _value,
      );

I/flutter ( 4147): 'package:flutter/src/material/dropdown.dart': Failed assertion: 'items == null ||
I/flutter ( 4147): value == null || items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not
I/flutter ( 4147): true.

 DropdownButton _hintDown() => DropdownButton<String>(
        items: [
          DropdownMenuItem<String>(
            value: "1",
            child: Text(
              "First",
            ),
          ),
          DropdownMenuItem<String>(
            value: "2",
            child: Text(
              "Second",
            ),
          ),
        ],
        onChanged: (value) {
          print("value: $value");
        },
        hint: Text(
          "Please select the number!",
          style: TextStyle(
            color: TEXT_BLACK,
          ),
        ),
      );

DropdownButton _hint2Down() => DropdownButton<String>(
        items: null,
        onChanged: null,
        disabledHint: Text("You can't select anything."),
      );

DropdownButton _normal2Down() => DropdownButton<String>(
        items: [
          DropdownMenuItem<String>(
            value: "1",
            child: Text(
              "First",
            ),
          ),
          DropdownMenuItem<String>(
            value: "2",
            child: Text(
              "Second",
            ),
          ),
        ],
        onChanged: (value) {
          setState(() {
            _value = value;
          });
        },
        value: _value,
        elevation: 2,
        style: TextStyle(color: PURPLE, fontSize: 30),
        isDense: true,
        iconSize: 40.0,
      );