[Need Help]upon receiving a FCM notification a popup would show in app flutter

[Anoirwork]

So as the title describe it i just need to know how to detect if an FCM notification has arrived and right away call a function where it will AlertDialog widget which will show to the user some message and give them the ability to choose between two things.
So i know that upon getting a FCM notification it fires onMessage i've put this function button nothing happen

Widget popUp() {
return AlertDialog(
  title: Container(
    child: const Icon(
      Icons.notifications_none,
      color: Colors.greenAccent,
      size: 40,
    ),
    alignment: Alignment.topLeft,
  ),
  content: Container(
      height: MediaQuery.of(context).size.width * 0.5,
      child: Column(
        children: <Widget>[
          const Text(
            "This is a pop up upon getting a notification",
            textAlign: TextAlign.left,
            style: TextStyle(fontSize: 18),
          ),
          const SizedBox(height: 20.0),
          const Text(
            "This is a pop up upon getting a notification",
            textAlign: TextAlign.left,
            style: TextStyle(fontSize: 16),
          ),
          Expanded(
              child: Align(
                  alignment: Alignment.bottomRight,
                  child: Row(
                    mainAxisAlignment: MainAxisAlignment.end,
                    children: <Widget>[
                      InkWell(
                          onTap: () => {launchFbPage()},
                          child: Wrap(children: [
                            const Text(
                              "Watch live",
                              textAlign: TextAlign.center,
                              style: TextStyle(
                                  fontSize: 16, color: Colors.blue),
                            ),
                            const Icon(
                              Icons.arrow_right,
                              color: Colors.blue,
                              size: 30.0,
                            )
                          ]))
                    ],
                  )))
        ],
      )),
);

Thanks for your advises stay healthy!