title: Can't create ExpansionPanelList with Items in Flutter
body: <p>I'm new to Flutter so i am trying to get into it. But I'm hanging on creating an ExpansionPanelList with ExpansionPanels in it. And Like the title says all created in googles Flutter. </p>
  
  <p>My code so far:</p>
  
  <pre><code>import 'package:flutter/material.dart';
  
  
  class ShoppingBasket extends StatefulWidget {
    @override
    ShoppingBasketState createState() =&gt; new ShoppingBasketState();
  }
  
  class ShoppingBasketState extends State&lt;ShoppingBasket&gt; {
  
    @override
    Widget build(BuildContext context) {
      return new ExpansionPanelList(
        children: &lt;ExpansionPanel&gt;[
          new ExpansionPanel(
            headerBuilder: _headerBuilder,
            body: new Container(
              child: new Text("body"),
            ),
          )
        ],
      );
    }
  
  
    Widget _headerBuilder(BuildContext context, bool isExpanded) {
      return new Text("headerBuilder");
    }
  }
  </code></pre>
  
  <p>But when I open the app the debugger says:</p>
  
  <blockquote>
    <p>Another exception was thrown: 'package:flutter/src/rendering/box.dart': Failed assertion: line 1430 pos 12: 'hasSize': is not true.</p>
  </blockquote>