I get my Firebase user Uid but its dataSnapshot keys value return null

My Firebase Realtime Database structure

I've made two apps in a eBook project, one for the admin and the other for the users.

When admin gives input to a book's data on Firebase then the user will be able to see the information provided by admin in his app.

I did this on the user app but the admin Id still has the following key's value is null.

And when I was debugging the app, my DataSnapshot showed me the value null of the key.

My app debugging mode:

when I get admin Uid but it show me null

when I set admin id in database Reference and its get Uid

and I'm get admin Id throw database reference but it dataSnapshot key's value is null

Here is my code:

private String adminId;
private Button button1,button2,button3;
private RecyclerView bookListRv;
private ArrayList<Book> bookList;
private BookAdapter bookAdapter;
private FirebaseAuth firebaseAuth;
private DatabaseReference databaseReference;

public BookListFragment() {
    // Required empty public constructor
}


@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    View view = inflater.inflate(R.layout.fragment_book_list, container, false);

    init(view);
    adminId = firebaseAuth.getUid();


    configRv();

    getBooks();


    return view;
}



private void init(View view) {

    bookListRv = view.findViewById(R.id.bookRVId);
    firebaseAuth = firebaseAuth.getInstance();
    databaseReference = FirebaseDatabase.getInstance().getReference();
    bookList = new ArrayList<>();
    bookAdapter = new BookAdapter(bookList, getContext());
}

private void configRv() {
    bookListRv.setLayoutManager(new LinearLayoutManager(getContext()));
    bookListRv.setAdapter(bookAdapter);
}

private void getBooks(){
    DatabaseReference showBookRef = databaseReference.child("Books").child(adminId);

    showBookRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            if (dataSnapshot.exists()) {
                for (DataSnapshot data:dataSnapshot.getChildren()){
                    Book book = data.child("BookInfo").getValue(Book.class);
                    bookList.add(book);
                    bookAdapter.notifyDataSetChanged();
                }
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });


}

Any help me for appreciated