Can't use any plugin inside FCM onBackgroundMessage (Flutter)

In my Flutter app, I'm trying to display a full screen Kotlin/Java activity when a FCM message is received in the background using a method channel. I've also tried to use plugins (flutter_local_notifications and CallKeep and GetX) but everytime my onBackgroundMessage is fired, I get a MissingPluginException.

FYI, I also register my plugins in my kotlin application:

override fun onCreate() {
    super.onCreate()
    FlutterFirebaseMessagingService.setPluginRegistrant(this)
}

override fun registerWith(registry: PluginRegistry?) 
{
    
    if (!registry!!.hasPlugin("com.rmawatson.flutterisolate.FlutterIsolatePlugin")) 
    {
        FlutterIsolatePlugin.registerWith(registry?.registrarFor("com.rmawatson.flutterisolate.FlutterIsolatePlugin")
    }

    if (!registry!!.hasPlugin("co.doneservices.callkeep.CallKeepPlugin")) 
    {
        CallKeepPlugin.registerWith(registry?.registrarFor("co.doneservices.callkeep.CallKeepPlugin"))
    }

    if (!registry!!.hasPlugin("io.flutter.plugins.firebasemessaging")) 
    {
        FirebaseMessagingPlugin.registerWith(registry?.registrarFor("io.flutter.plugins.firebasemessaging.FirebaseMessagingPlugin"));
    }
}

And here is my Flutter code:

void callReceive(bool show) async {
  if (show == true) {
    _channelCallReceive.invokeMethod('display'); 
    isolateCallReceive.kill();
  }
}

Future<Map<String, dynamic>> myBackgroundMessageHandler(Map<String, dynamic> message) async 
{  
  isolateCallReceive = await FlutterIsolate.spawn(callReceive, true);
}

I've looked at many other solutions on SO but nothing worked for me. I've wasted A LOT of time on this.

The bottom line is... How can I display a full screen notification over the lock screen (or when the flutter app is in the background)?