Error in remoteMessage.getNotification().getBody()

Try to add a notification object on your $message. The body of your POST request must be something like:

{
    "to" : "aUniqueKey",
    "notification" : {
      "body" : "great match!",
      "title" : "Portugal vs. Denmark"
    },
    "data" : {
      "Nick" : "Mario",
      "Room" : "PortugalVSDenmark"
    }
}

Your remoteMessage.getNotification() returnsnull because the body of your POST request doesn't contain a notification object.

Use notifications when you want FCM to handle displaying a notification on your client app's behalf. Use data messages when you want your app to handle the display or process the messages on your Android client app, or if you want to send messages to iOS devices when there is a direct FCM connection.

Check the Documentation for Advanced Messaging Options for your reference.