Popup is not working in Flutter Web VIew package

I am using 'flutter_webview' package for payment the web view working fine. but on payment its open new popup, that popup is working in other external browser but not in mobile view.

please guide me to handle the popup in webview.

 Expanded(
              child: WebView(
                debuggingEnabled: true,
                initialUrl: paymentUrl,
                allowsInlineMediaPlayback: true,
                javascriptMode: JavascriptMode.unrestricted,
                gestureNavigationEnabled: true,
                onWebViewCreated: (WebViewController webViewController) {
                  _controller = webViewController;
                },
                onWebResourceError: (WebResourceError webviewerrr) {
                  print("Handle your Error Page here");
                },
                javascriptChannels: Set.from([
                  JavascriptChannel(
                    name: 'Alert',
                    onMessageReceived: (JavascriptMessage message) {
                      // alert message = Test alert Message
                      print(message.message);
                      // TODO popup
                    },
                  )
                ]),
              ),
            ),