Making spinner background transparent in flutter

I'm new to flutter and making my first webview app. Here I'm trying to add a spinner every time when a user tries to click the link or page load. I want to make spinner background opacity a bit low just like the given example, but opacity doesn't work at all.

My approach

   Container(
            width: MediaQuery.of(context).size.width,
            height: MediaQuery.of(context).size.height,
            color: Colors.white.withOpacity(0.5),
            child: Center(
              child: SpinKitDualRing(
                color: Colors.pinkAccent,
                size: 45.0,
                controller: AnimationController(
                  vsync: this,
                  duration: const Duration(milliseconds: 1200),
                ),
              ),
            ),
          )

I'm using here flutter_spinkit package as a spinner.

Full code

// ignore_for_file: prefer_const_constructors
// ignore: use_key_in_widget_constructors
import 'dart:async';
import 'dart:io';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:webview_flutter/webview_flutter.dart';
import 'package:flutter_spinkit/flutter_spinkit.dart';
import 'package:splash_screen_view/SplashScreenView.dart';

void main(){
  SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle(
    statusBarColor: Color(0xff1e2229)
  ));
   runApp(MyApp());
}

 
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
     
        Widget spalshfirst = SplashScreenView(
              navigateRoute: WebViewClass(),
              duration: 3000,
              imageSize: 80,
              imageSrc: 'assets/splash.png',
              text: "Food Delivery",
              textType: TextType.TyperAnimatedText,
              textStyle: TextStyle(
                fontSize: 25.0,
              ),
              colors: const [
                Colors.purple,
                Colors.blue,
                Colors.yellow,
                Colors.red,
              ],
              backgroundColor: Colors.white,
            );

    return  MaterialApp(
            debugShowCheckedModeBanner: false,
            home:  Scaffold(
                       body: spalshfirst
                      )
              );
  
  }
}
 
class WebViewClass extends StatefulWidget {
  WebViewState createState() => WebViewState();
}

class WebViewState extends State<WebViewClass> with TickerProviderStateMixin{

 late WebViewController _controller;

  final Completer<WebViewController> _controllerCompleter =
      Completer<WebViewController>();
  //Make sure this function return Future<bool> otherwise you will get an error
  Future<bool> _onWillPop(BuildContext context) async {
    if (await _controller.canGoBack()) {
      _controller.goBack();
      return Future.value(false);
    } else {
      return Future.value(true);
    }
  }

  @override
   void initState() {
     super.initState();
         // Enable hybrid composition.
    if (Platform.isAndroid) WebView.platform = SurfaceAndroidWebView();
   }

  bool isLoading = false;
  final key = UniqueKey();
  
 int position = 0;
  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: () => _goBack(context),
      child: Scaffold(
        resizeToAvoidBottomInset: false,
        appBar: null,
        body: SafeArea(
          child: IndexedStack(
            index: position,
            children: [
              WebView(
                initialUrl: 'https://google.com',
                javascriptMode: JavascriptMode.unrestricted,
                key: key,
                onPageStarted: (value) {
                  setState(() {
                    position = 1;
                  });
                },
                onPageFinished: (value) {
                  setState(() {
                    position = 0;
                  });
                },
                onWebViewCreated: (WebViewController webViewController) {
                  _controllerCompleter.future
                      .then((value) => _controller = value);
                  _controllerCompleter.complete(webViewController);
                },
              ),
              Container(
                width: MediaQuery.of(context).size.width,
                height: MediaQuery.of(context).size.height,
                color: Colors.white.withOpacity(0.5),
                child: Center(
                  child: SpinKitDualRing(
                    color: Colors.pinkAccent,
                    size: 45.0,
                    controller: AnimationController(
                      vsync: this,
                      duration: const Duration(milliseconds: 1200),
                    ),
                  ),
                ),
              )
            ],
          ),
        ),
      ),
    );
  }

 Future<bool> _goBack(BuildContext context) async {
    if (await _controller.canGoBack()) {
      _controller.goBack();
      return Future.value(false);
    } else {
      showDialog(
          context: context,
          builder: (context) => AlertDialog(
                title: Text('Do you want to exit from Foodrive?'),
                actions: <Widget>[
                  TextButton(
                    onPressed: () {
                      Navigator.of(context).pop();
                    },
                    child: Text('No'),
                  ),
                  TextButton(
                    onPressed: () {
                      SystemNavigator.pop();
                    },
                    child: Text('Yes'),
                  ),
                ],
              ));
      return Future.value(true);
    }
  } 


}