How to use Paywall in Flutter? (Android part in kotlin)

Hi, i have a flutter app and want to include the new paywall feature, but i get error messages, i cant make it work. (Currently i can only test for android, if thats important)

If i leave my MainActivity.kt (the main activity for android is in kotlin, not java!) at:

class MainActivity : FlutterActivity() { }

I get the error: 

Unhandled Exception: PlatformException(0, Make sure your MainActivity inherits from FlutterFragmentActivity, null, null)

when i click the button that has this onTap:

onTap: () async {  await RevenueCatUI.presentPaywall();},

But if i change it to:

package com.mypackage

import io.flutter.embedding.android.FlutterActivity
import com.revenuecat.purchases_flutter.FlutterFragmentActivity

class MainActivity : FlutterFragmentActivity() {
} 

like the documents and community suggests, it gets Unresolved reference: FlutterFragmentActivity

and the app doesnt even start.

What should i do to solve this problem? Kotlin seems to be the standard way for Android in Flutter, and i dont think i can change it… 

Thanks and best wishes, Janne