AppCheck - no MainActivity.java file in project as per install instructions

[Zelfapp]

Install instructions mention a MainActivity.java file to update. However, my Flutter project has no MainActivity.java. The instructions need to be updated for how to handle this situation. Where do I put the MainActivity.java file if no file exists or do I update MainActivity.kt that does exist, and if so what is the code for that?

I've tried running flutter create . as many SO posts have suggested, but this does not create a MainActivity.java file in the project.

The contents of my android/app/src/main/kotlin/com/mysite/myapp/MainActivity.kt file:

package com.mysite.myapp

import io.flutter.embedding.android.FlutterActivity

class MainActivity: FlutterActivity() {
}