addOnCompleteListener documentReference变量显示空值

Question

database snapshot

我正在使用Android Studio连接Firestore和Android应用。如何在侦听器调用后将数据存储在侦听器中以使用，但始终显示null值。似乎addOnCompleteListener进入了后台，documentReference变量首先使用了如何解决它。 和
documentReference变量显示空值

    DocumentReference documentReference;
FirebaseFirestore db = FirebaseFirestore.getInstance();
db.collection("CustomerUser")
                .get()
                .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                        if (task.isSuccessful()) {
                            for (QueryDocumentSnapshot document : task.getResult()) {

                                documentReference = document.getDocumentReference("businessUsedID");
                                Log.d("REF_DATA", document.getId() + " => " + documentReference.getId());
                            }
                        } else {
                            Log.w("REF_DATA", "Error getting documents.", task.getException());
                        }
                    }
                });
documentReference.addSnapshotListener(this, new EventListener<DocumentSnapshot>() {
            @Override
            public void onEvent(@Nullable DocumentSnapshot documentSnapshot, @Nullable FirebaseFirestoreException e) {
                Log.i("REF_DATA", "onEvent: "+documentSnapshot.getString("PaymentMode"));
            }
        });