title: Error when adding container color: Incorrect use of ParentDataWidget
body: <p>I'm trying to add a background color to my container, then maybe add some border radius with BoxDecoration(). When adding color: Colors.red to the container, it's throwing out:</p>
  <blockquote>
  <p>Incorrect use of ParentDataWidget. Expanded widget must be placed directly inside Flex widgets.</p>
  </blockquote>
  <pre class="lang-dart prettyprint-override"><code>Container(
    color: Colors.red,
    child: Expanded(
      child: ListView.builder(
        padding: EdgeInsets.all(0.0),
        itemCount: _clients.length,
        itemBuilder: (context, index) {
          final client = _clients[index];
  
          return Dismissible(
            key: Key(client),
            direction: DismissDirection.startToEnd,
            onDismissed: (direction) {
              setState(() {
                _clients.removeAt(index);
              });
            },
            background: Container(color: Color(0xff171e24)),
            child: _clientListTile(context, client)
          );
        }
      ),
    ),
  )
  </code></pre>