这可以解决问题，但也可以有更好的解决方案。 （感谢@ user1462442，来自上面的评论。）

This solves the problem, but there could be better solutions as well. (Thanks to @user1462442 from the comments above.)

@override
Widget build(BuildContext context) {
 if (!_isReady) return new Container();
 if (!controller.value.initialized) return new Container();

 return new Scaffold(
   body: new Container(
     child: new AspectRatio(
       aspectRatio: controller.value.aspectRatio,
       child: new CameraPreview(controller),
     ),
   ),
   floatingActionButton: new FloatingActionButton(
     onPressed: _isReady ? capture : null,
     child: const Icon(
       Icons.camera,
       color: Colors.white,
     ),
   ),
   floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,
 );
}