Flutter flatter:webview父对象

我正在尝试为我的Flatter webview页面添加加载指示器。我正在使用，但我从下面的代码中得到一个错误

引发了另一个异常：ParentDataWidget的使用不正确。

import 'package:webview_flutter/webview_flutter.dart';

return Container(
  child: Scaffold(
    appBar: AppBar(
      title:Text("Loading..."),
      centerTitle: true,
    ),
    backgroundColor: Colors.transparent,
    body: Container(
      child: Expanded(
        child: Stack(
          children: [
            WebView(
              initialUrl: "https://google.com/",
              javascriptMode: JavascriptMode.unrestricted,
              onWebViewCreated: (WebViewController webViewController) {
                _controller.complete(webViewController);
              },
              onPageStarted: (String url) {
                print('Page started loading: $url');
              },
              onPageFinished: (String url) {
                print('Page finished loading: $url');
              },
              gestureNavigationEnabled: true,
            ),
            Positioned(
              child: CircularProgressIndicator(),
              left: (MediaQuery.of(context).size.width / 2),
              top: (MediaQuery.of(context).size.height / 2),
            ),
            Center(
              child: CircularProgressIndicator(),
            )
          ],
        ),
      ),
    )
  ),
);