title: Flutter - DropdownButton value stays empty after selection
body: <p>Suppose we have created a simple DropdownButton <code>ABCPageState</code> in an ABC stateful widget. </p>
  
  <pre><code>class ABCPageState extends State&lt;ABCPage&gt; {
  
    @override 
    Widget build(BuildContext context) {
      return new Scaffold(
        body: new Container(
          child:
            new DropdownButton(
                  hint: new Text("Please select value"),
                  items: &lt;String&gt;['Value 1', 'Value2'].map((String value) {
                    return new DropdownMenuItem&lt;String&gt;(
                      value: value,
                      child: new Text(value),
                    );
                  }).toList(),
                  onChanged: (_) {},
            )
        )
      );
    }
  }
  </code></pre>
  
  <p>but <strong>NO VALUE is selected</strong> after we click on one of the options. In other words - DropdownButton is empty even though we clicked an item. How can we fix that?</p>