title: Reset value on dropdown (Flutter)
body: <p>I have two dropdown which is states and cities, basically when user choose a state, it will automatically set the value of cities for the states. When i choose the state, the dropdown for cities appear, and after i choose the cities, if i want to change the state back, it will give error</p>
  
  <p>"Another exception was thrown: 'package:flutter/src/material/dropdown.dart': Failed assertion: line 513 pos 15: 'items == null || value == null || items.where((DropdownMenuItem item) => item.value == value).length == 1': is not true."</p>
  
  <pre><code>List state = [
  "Kuala Lumpur",
  "Selangor",
  "Johor",
  "Kedah",
  "Kelantan",
  "Melaka",
  "Negeri Sembilan",
  "Pahang",
  "Penang",
  "Perak",
  "Perlis",
  "Sabah",
  "Sarawak",
  "Terengganu"
  
   ];
  
  List kl = [
  "Ampang Hilir",
  "Bandar Damai Perdana",
  "Bandar Menjalara",
  "Bandar Tasik Selatan",
  "Bangsar",
  "Bangsar South",];
  
  List sel = [
  "Ampang",
  "Ara Damansara",
  "Balakong",
  "Bandar Bukit Raja",
  "Bandar Kinrara",
  "Bandar Puteri Puchong",
  "Bandar Sunway",
  "Bandar Utama",];
  
  @override
  
  
  void initState() {
      super.initState();
  
  _dropDownMenuStates = getDropDownMenuState();
  
  }
  
  List&lt;DropdownMenuItem&lt;String&gt;&gt; getDropDownMenuState() {
  List&lt;DropdownMenuItem&lt;String&gt;&gt; state1 = new List();
  for (String statelist in state) {
    state1.add(
        new DropdownMenuItem(value: statelist, child: new Text(statelist)));
  }
  return state1;
  
  
  }
  
  List&lt;DropdownMenuItem&lt;String&gt;&gt; getDropDownMenuKL() {
  List&lt;DropdownMenuItem&lt;String&gt;&gt; kl1 = new List();
  for (String kllist in kl) {
    kl1.add(new DropdownMenuItem(value: kllist, child: new Text(kllist)));
  }
  return kl1;
  
  
   }
  
    List&lt;DropdownMenuItem&lt;String&gt;&gt; getDropDownMenuSEL() {
      List&lt;DropdownMenuItem&lt;String&gt;&gt; sel1 = new List();
      for (String sellist in sel) {
        sel1.add(new DropdownMenuItem(value: sellist, child: new Text(sellist)));
      }
      return sel1;
    }
  
      Expanded(
    child: PhysicalModel(
        borderRadius:
            new BorderRadius.circular(50.0),
        color: Colors.white,
        child: new Container(
            padding: EdgeInsets.only(
                left: 10.0, right: 10.0),
            height: 40.0,
            decoration: new BoxDecoration(
                borderRadius:
                    new BorderRadius
                        .circular(50.0),
                border: new Border.all(
                  width: 3.0,
                  color: Colors.grey[300],
                )),
            child: new FittedBox(
              fit: BoxFit.contain,
              child: DropdownButton(
                hint: new Text(
                    allTranslations
                        .text('city')),
                value: _currentCity,
                items: _dropDownMenuCity,
                onChanged:
                    changedDropDownCity,
              ),
            ))),
  ),
  
  SizedBox(
    width: 10.0,
  ),
  Expanded(
    child: PhysicalModel(
        borderRadius:
            new BorderRadius.circular(50.0),
        color: Colors.white,
        child: new Container(
            padding: EdgeInsets.only(
                left: 10.0, right: 10.0),
            height: 40.0,
            decoration: new BoxDecoration(
                borderRadius:
                    new BorderRadius
                        .circular(50.0),
                border: new Border.all(
                  width: 3.0,
                  color: Colors.grey[300],
                )),
            child: new FittedBox(
              fit: BoxFit.contain,
              child: DropdownButton(
                hint: new Text(
                    allTranslations
                        .text('state')),
                value: _currentState,
                items: _dropDownMenuStates,
                onChanged:
                    changedDropDownState,
              ),
            ))),
  ),
  
  void changedDropDownState(String selectedState) {
  setState(() {
    _currentState = selectedState;
    if (selectedState.toString() == "Kuala Lumpur") {
      _dropDownMenuCity = getDropDownMenuKL();
    } else if (selectedState.toString() == "Selangor") {
      _dropDownMenuCity = getDropDownMenuSEL();}
  
  
  });
  
  
  }
  
    void changedDropDownCity(String selectedCity) {
      setState(() {
        _currentCity = selectedCity;
      });
    }
  </code></pre>