title: FormBuilderCheckboxList internal value is reset but the checkbox still in checked condition (not cleared) if using INITIAL VALUE
body: <p>I am using <a href="https://pub.dev/packages/flutter_form_builder#-readme-tab-" rel="nofollow noreferrer">FLUTTER_FORM_BUILDER</a> package for my custom form. I build checkbox list using FormBuilderCheckbox, i give it initial value using <code>initialValue</code> construtor. The problem occur when I'm trying to clear the checkbox. I use globalkey.currentState.reset() to reset the value. It does reset the internal value of the checkbox, but it seems the checkboxes still in Checked Condition.</p>
  
  <p>How can i clear it? I can't use .clear() since i can't assign controller to the FormBuilderCheckbox.</p>
  
  <p>Any insight would be appreciated, thank you.</p>
  
  <p>EDIT: This is a simplified code to reproduce.</p>
  
  <pre><code>import 'package:flutter/material.dart';
  import 'package:flutter_form_builder/flutter_form_builder.dart';
  
  void main() =&gt; runApp(MyApp());
  
  class MyApp extends StatefulWidget {
    @override
    _MyAppState createState() =&gt; _MyAppState();
  }
  
  class _MyAppState extends State&lt;MyApp&gt; {
    Map _initialData = {
      'checkbox': ['1'],
    };
  
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: MyCustomForm(data: _initialData),
      );
    }
  }
  
  class MyCustomForm extends StatefulWidget {
    final Map data;
  
    const MyCustomForm({Key key, @required this.data}) : super(key: key);
  
    @override
    _MyCustomFormState createState() =&gt; _MyCustomFormState();
  }
  
  class _MyCustomFormState extends State&lt;MyCustomForm&gt; {
    List _checkboxInitial;
    final GlobalKey&lt;FormBuilderState&gt; _fbKey = GlobalKey&lt;FormBuilderState&gt;();
  
    @override
    void initState() {
      setState(() {
        _checkboxInitial = List.from(widget.data['checkbox']);
      });
      super.initState();
    }
  
    @override
    Widget build(BuildContext context) {
      return Material(
        child: Container(
          height: 200,
          width: 200,
          child: FormBuilder(
            key: _fbKey,
            child: Column(
              children: &lt;Widget&gt;[
                FormBuilderCheckboxList(
                  initialValue: _checkboxInitial,
                  decoration: InputDecoration(border: InputBorder.none),
                  attribute: 'checkbox',
                  options: [
                    '1',
                    '2',
                    '3',
                  ]
                      .map(
                        (data) =&gt; FormBuilderFieldOption(
                          child: Text(data),
                          value: data,
                        ),
                      )
                      .toList(growable: false),
                ),
                RaisedButton(
                  onPressed: () {
                    setState(() {
                      _fbKey.currentState.reset();
                    });
                  },
                  child: Text('Clear'),
                ),
              ],
            ),
          ),
        ),
      );
    }
  }
  </code></pre>