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

Question

it shows this error.

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

main.dart

import 'package:flutter/material.dart';
import 'util/conver_util.dart';

void main() {
  runApp(MaterialApp(
    title: 'Measures Converter',
    home: MyApp(),
  ));
}

class MyApp extends StatefulWidget {
  @override
  MyAppState createState() => MyAppState();
}

class MyAppState extends State<MyApp> {
  double _numberFrom = 0;
  String _startMeasure = "";
  String _convertedMeasure = "";
  double _result = 0;
  String _resultMessage = "";
  @override
  Widget build(BuildContext context) {
    double sizeX = MediaQuery.of(context).size.width;
    double sizeY = MediaQuery.of(context).size.height;
    final TextStyle inputStyle = TextStyle(
      fontSize: 20,
      color: Colors.blue[900],
    );
    final TextStyle labelStyle = TextStyle(
      fontSize: 24,
      color: Colors.grey[700],
    );

    final spacer = Padding(padding: EdgeInsets.only(bottom: sizeY/40));
    final List<String> _measures = [
      'meters',
      'kilometers',
      'grams',
      'kilograms',
      'feet',
      'miles',
      'pounds (lbs)',
      'ounces',
    ];
    return Scaffold(
      appBar: AppBar(
        title: Text('Measures Converter'),
      ),
      body: Container(
        width: sizeX,
        padding: EdgeInsets.all(sizeX/20),
        child: SingleChildScrollView(child: Column(
          children: [
            Text('Value', style: labelStyle,),
            spacer,
            TextField(
              style: inputStyle,
              decoration: InputDecoration(
                hintText: "Please insert the measure to be converted",
              ),
              onChanged: (text) {
                setState(() {
                  _numberFrom = double.parse(text);
                });
              },
            ),
            spacer,
            Text('From', style: labelStyle,),
            spacer,
            DropdownButton(
              isExpanded: true,
              style: inputStyle,
              value: _startMeasure,
              items: _measures.map((String value) {
                return DropdownMenuItem<String>(
                  value: value,
                  child: Text(value, style: inputStyle,),
                );
              }).toList(),
              onChanged: (value) {
                onStartMeasureChanged(value!);
              },
            ),
            spacer,
            Text('To', style: labelStyle,),
            spacer,
            DropdownButton(
              isExpanded: true,
              style: inputStyle,
              value: _convertedMeasure,
              items: _measures.map((String value) {
                return DropdownMenuItem<String>(
                  value: value,
                  child: Text(value, style: inputStyle,),
                );
              }).toList(),
              onChanged: (value) {
                onConvertedMeasureChanged(value!);
              },
            ),
            spacer,
            RaisedButton(child:Text('Convert', style: inputStyle),
              onPressed: ()=>convert(),),
            spacer,
            Text(_resultMessage, style: labelStyle,)
          ],
        )),
      ),
    );
  }

  void onStartMeasureChanged(String value) {
    setState(() {
      _startMeasure = value;
    });
  }
  void onConvertedMeasureChanged(String value) {
    setState(() {
      _convertedMeasure = value;
    });
  }

  void convert() {
    if (_startMeasure.isEmpty || _convertedMeasure.isEmpty || _numberFrom==0) {
      return;
    }
    Conversion c = Conversion();
    double result = c.convert(_numberFrom, _startMeasure, _convertedMeasure);
    setState(() {
      _result = result;
      if (result == 0) {
        _resultMessage = 'This conversion cannot be performed';
      }
      else {
        _resultMessage = '${_numberFrom.toString()} $_startMeasure are ${_result.toString()} $_convertedMeasure';
      }

    });
  }

  RaisedButton({required Text child, required void Function() onPressed}) {}

}

conver_util.dart

class Conversion {
  //helps transform the strings in numbers for the List
  final int w = 8;
  var formulas;
  Map<String, int> measures = {
    'meters' : 0,
    'kilometers' : 1,
    'grams' : 2,
    'kilograms' : 3,
    'feet' : 4,
    'miles' : 5,
    'pounds (lbs)' : 6,
    'ounces' : 7,
  };
  //builds the list containing the formulas
  //List<List<double>> formulas;
  Conversion() {
    formulas = {
      '0':[1,0.001,0,0,3.28084,0.000621371,0,0],
      '1':[1000,1,0,0,3280.84,0.621371,0,0],
      '2':[0,0,1,0.0001,0,0,0.00220462,0.035274],
      '3':[0,0,1000,1,0,0,2.20462,35.274],
      '4':[0.3048,0.0003048,0,0,1,0.000189394,0,0],
      '5':[1609.34, 1.60934,0,0,5280,1,0,0],
      '6':[0,0,453.592,0.453592,0,0,1,16],
      '7':[0,0,28.3495,0.0283495,3.28084,0,0.0625, 1],
    };
  }

  double convert(double value, String from, String to) {
    int? nFrom = measures[from];
    int? nTo = measures[to];
    var multiplier = formulas[nFrom.toString()][nTo];
    return value * multiplier;
  }
}

Answer 1

values ​​must be one of the items in the _measures list.For example:

     DropdownButton(
          isExpanded: true,
          style: inputStyle,
          value: 'meters',
          items: _measures.map((String value) {
            return DropdownMenuItem<String>(
              value: value,
              child: Text(value, style: inputStyle,),
            );
          }).toList(),
          onChanged: (value) {
            onStartMeasureChanged(value!);
          },
        ),