currentUser() is not working type why I am seeing the expression doesn't evaluate to a function, so it can't be invoked

I'm new in Flutter, I'm working on a simple authentication app when I registered i should navigate to the chat screen and i want to grab current user but when i call FireBaseAuth.instance.currentUser() I got an exception "the expression doesn't evaluate to a function, so it can't be invoked." Why I'm seeing this? Please Help

import 'package:firebase_auth/firebase_auth.dart';
import 'package:flutter/material.dart';
import 'package:flash_chat/constants.dart';

class ChatScreen extends StatefulWidget {
  static const String id = "chatScreen";
  @override
  _ChatScreenState createState() => _ChatScreenState();
}

class _ChatScreenState extends State<ChatScreen> {
  final FirebaseAuth _auth = FirebaseAuth.instance;
  // ignore: deprecated_member_use
  FirebaseUser loginUser;

  @override
  void initState() {
    super.initState();
    getCurrentUser();
  }

  void getCurrentUser() async {
// HERE I got an Exception "The expression doesn't evaluate to a function, so it can't be invoked."
    final user = await _auth.currentUser();
    if (user != null) {
      loginUser = user;
      print(loginUser.email);
    }
  }