title: flutter checkbox doesn't work
body: <p>I want to build a checkbox with <code>CheckboxListTile</code> inside this widget dialog but when I tap the checkbox the <code>checked</code> on the checkbox doesn't change.</p>
  
  <p>This is my code:</p>
  
  <pre><code>Future&lt;Null&gt; _showGroupDialog(BuildContext context) async {
      await showDialog(
          context: context,
          builder: (BuildContext dialogContext) =&gt;
              Dialog(child: _buildCheckboxGroups(context)));
  }
  
  Widget _buildCheckboxGroups(BuildContext context) {
      List&lt;Widget&gt; childrens = List.generate(_groups.length, (index) {
        return CheckboxListTile(
          title: Text(_groups[index].name),
          value: _groups[index].checked,
          onChanged: (bool val) {
            setState(() {
              _groups[index].checked = val;
            });
          },
        );
      });
  
      return Container(
          child: Column(
        mainAxisSize: MainAxisSize.min,
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: childrens,
      ));
  }
  </code></pre>
  
  <p>Btw, the <code>onChange</code> method is invoked when I tap the checkbox. Can anyone solve this?</p>