Default FirebaseApp is not initialized in this process com.example.visitorsapp. Make sure to call FirebaseApp.initializeApp(Context) first.

Example: default firebaseapp is not initialized in this process

I had this same issue some time ago.

You're trying to get an instance of Firebase without initialize it. Please add this line of code before you try to get an instance of Firebase, in your main function or a FutureBuilder:

FirebaseApp.initializeApp();