title: setState does not update GridView in flutter
body: <p>I have read the other questions and while they do have similar titles to this one, they don't cover my issue. Please read below.</p>
  <p>I'm using Android Studio. I have a simple stateful app with a GridView. Its children are in a list &quot;geza&quot;. When I press the button a Text('Hello') is added to the list.</p>
  <p>This is done inside the setState() method. The newly added widget will not appear in the grid.</p>
  <p>If I do a hot-redeploy from Android studio, then after the redeploy the widget will show up. So it looks like the widget is added to the list, but the Grid is not updated.</p>
  <p>What am I doing wrong?</p>
  <pre><code>class MyHomePage extends StatefulWidget {
    MyHomePage({Key? key}) : super(key: key);
  
    @override
    _MyHomePageState createState() =&gt; _MyHomePageState();
  }
  
  class _MyHomePageState extends State&lt;MyHomePage&gt; {
  
    var geza = &lt;Widget&gt;[ Text('AAA') ];
  
    @override
    Widget build(BuildContext context) {
      return Scaffold(
        appBar: AppBar(
          title: Text('My Test'),
        ),
        body: Center(
          child: GridView.count(crossAxisCount: 2,
            children: this.geza
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            setState(() {
              this.geza.add(Text('Hello'));
            });
          },
          child: Icon(Icons.add),
        ),
      );
    }
  }
  </code></pre>