build.dart

 Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Call Center'),
      ),
      body: StreamBuilder(
          stream: callCenter.changes,
          builder: (context, snapshot) {
            return Column(
              children: _buildBody() +
                  [_buildButton()] +
                  [_buildQueueView(callCenter.queueCalls)],
            );
          }),
      bottomNavigationBar: BottomAppBar(
        child: Container(
          height: 50.0,
        ),
      ),
    );
  }