on a null object reference :Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()'

Question

1. Logcat

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.google.firebase.auth.FirebaseUser.getUid()' on a null object reference at com.example.medcareinapp.UserSideScreens.PatientDashboardActivity.onCreate(PatientDashboardActivity.java:89)

2. PatientDashboardActivity.java

private static FirebaseAuth mAuth; public static String UserName;

DrawerLayout drawerLayout;
TextView nav_name;
CardView  bookRideCard, profileCard, activeRideCard, historyCard, book_doctor, active_appointments;
Intent i;

FirebaseFirestore firebaseFirestore;
FirebaseAuth firebaseAuth;
String currentUserKey;




private int PERMISSION_CODE = 1;

//shared prefs variables
public static final String SHARED_PREFS = "SharedPrefs";
public static final String userType = "userType";
public static final String userNodeKey = "userKey";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_patient_dashboard);



    // check if the permissions are granted
    if (ActivityCompat.checkSelfPermission(getApplicationContext(), Manifest.permission.ACCESS_FINE_LOCATION) != PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(getApplicationContext(), Manifest.permission.ACCESS_COARSE_LOCATION) != PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(PatientDashboardActivity.this, new String[]{Manifest.permission.ACCESS_FINE_LOCATION, Manifest.permission.ACCESS_COARSE_LOCATION}, PERMISSION_CODE);
        Toast.makeText(getApplicationContext(), "Provide the permissions to continue", Toast.LENGTH_SHORT).show();
        return;
    }

    SharedPreferences sharedPreferences = getSharedPreferences(SHARED_PREFS, MODE_PRIVATE);
    SharedPreferences.Editor editor = sharedPreferences.edit();

    firebaseAuth = FirebaseAuth.getInstance();
    firebaseFirestore = FirebaseFirestore.getInstance();
    currentUserKey = firebaseAuth.getCurrentUser().getUid();

    mAuth = FirebaseAuth.getInstance();
    UserName = getIntent().getStringExtra("UserName");

    drawerLayout = findViewById(R.id.drawer);
    nav_name = findViewById(R.id.nav_name);
    nav_name.setText(UserName);

    book_doctor = findViewById(R.id.book_doctor);
    active_appointments = findViewById(R.id.active_appointments);
    bookRideCard = findViewById(R.id.bookRideCardPDashboard);