Application.kt error Unresolved reference: setPluginRegistrant - firebase

Question

I get Unresolved reference: setPluginRegistrant error when I try to add the below code in the Application.kt file:
Code causing the issue:
FlutterFirebaseMessagingBackgroundService.setPluginRegistrant(this);
My Application.kt file:
package com.application.test
import io.flutter.app.FlutterApplication
import io.flutter.embedding.engine.FlutterEngine
import io.flutter.plugin.common.PluginRegistry
import io.flutter.plugins.GeneratedPluginRegistrant
import io.flutter.plugins.firebase.messaging.FlutterFirebaseMessagingBackgroundService;
class Application : FlutterApplication(), PluginRegistry.PluginRegistrantCallback {
override fun onCreate() {
super.onCreate()
FlutterFirebaseMessagingBackgroundService.setPluginRegistrant(this); //this line is the issue
}
override fun registerWith(registry: PluginRegistry?) {
GeneratedPluginRegistrant.registerWith(FlutterEngine(this));
}
}
Without this line, the code compiles but I get the following leak via LeakCanary:
io.flutter.plugins.firebase.messaging.FlutterFirebaseMessagingBackgroundService instance
​ Leaking: YES (ObjectWatcher was watching this because io.flutter.plugins.firebase.messaging.
​ FlutterFirebaseMessagingBackgroundService received Service#onDestroy() callback)
In both cases the background messages work without a problem but I do also get the following messages in the Logcat:
W/FLTFireMsgService: A background message could not be handled in Dart as no onBackgroundMessage handler has been registered.
W/FirebaseMessaging: Notification Channel set in AndroidManifest.xml has not been created by the app. Default value will be used.
My AndroidManifest.xml:
<application
android:name="com.application.test.Application"
android:label="Test App"
android:icon="#mipmap/ic_launcher">
<activity
android:name="com.application.test.MainActivity"
android:launchMode="singleTop"
android:theme="#style/LaunchTheme"
<meta-data
android:name="io.flutter.embedding.android.NormalTheme"
android:resource="#style/NormalTheme"
/>
</activity>
<meta-data
android:name="flutterEmbedding"
android:value="2" />
<meta-data
android:name="com.google.firebase.messaging.default_notification_color"
android:resource="#color/notification_color" />
<meta-data
android:name="com.google.firebase.messaging.default_notification_channel_id"
android:value="#string/default_notification_channel_id" />
</application>
Any suggestion how to fix the issue?