Android studio app crashes when read the firestore document

Question

I try to make arraylist from the firestore. When I run the code the app got crashes. Do I need to add something here to make the app run? And how can I get all the document in firestore that have unique id?

public String load() {

    contentList = new ArrayList<>();
   
    contentList.clear();
    firebaseFirestore.collection("plans").document("W1ls1b0lYNw9hxyuJyJ5").collection("versions").get()
            .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                @Override
                public void onComplete(@NonNull Task<QuerySnapshot> task) {
                    if (task.isSuccessful()) {

                        for (QueryDocumentSnapshot doc : task.getResult()) {

                                String List = doc.getString("content");
                                contentList.add(List);





                        }
                        Log.d(TAG, "Length is:"+ " => " + contentList.size());
                        Log.d(TAG, "a : =>" + contentList.get(0));
                        Log.d(TAG, "b : =>" + contentList.get(1));
                        Log.d(TAG, "c : =>" + contentList.get(2));
                        Log.d(TAG, "d : =>" + contentList.get(3));


                    }else {
                        Log.e(TAG, "Firestore error");
                    }
                }

            });

    return contentList.get(0).toString();
    //contentList.get(0).toString()
}