執行我的應用程序時，我收到以下錯誤：java.lang.NullPointerException：無法為 child() 中的參數“pathString”傳遞 null但我之前沒有收到這個錯誤。有人可以幫助我嗎？請看錯誤控制臺，以及上面的java代碼消息片段.javaMessagesFragment 中的第一個錯誤，錯誤發生在 MessagesDbRef 中，但 firebase 不為空。public class MessagesFragment extends Fragment {    private static final String TAG = MessagesFragment.class.getSimpleName();    private FirebaseAuth mAuth;    private FirebaseUser user;    private DatabaseReference userDbRef;    private DatabaseReference messagesDbRef;    private DatabaseReference conversationsDbRef;    private FirebaseRecyclerAdapter firebaseRecyclerAdapter;    private RecyclerView recyclerViewConversations;    private Context context;    ImageView imgViewNoMessages;    TextView txtNoMessages;    private String currentUserId;    public MessagesFragment() {    }    @Override    public View onCreateView(@NonNull LayoutInflater inflater, ViewGroup container,                             Bundle savedInstanceState) {        View messagesView = inflater.inflate(R.layout.fragment_messages, container, false);        imgViewNoMessages = (ImageView) messagesView.findViewById(R.id.image_no_messages);        txtNoMessages = (TextView) messagesView.findViewById(R.id.text_no_messages);        this.context = getContext();        mAuth = FirebaseAuth.getInstance();        user = mAuth.getCurrentUser();        if (user != null) {            currentUserId = user.getUid();        }        recyclerViewConversations = (RecyclerView) messagesView.findViewById(R.id.rv_conversations);        userDbRef = FirebaseDatabase.getInstance().getReference().child("Users");        userDbRef.keepSynced(true);        conversationsDbRef = FirebaseDatabase.getInstance().getReference().child("Chat").child(currentUserId);        conversationsDbRef.keepSynced( true );        messagesDbRef = FirebaseDatabase.getInstance().getReference().child("Messages").child(currentUserId);        messagesDbRef.keepSynced(true);    }