use current user id as reference node in firebase database

I'm trying to get authenticated user id to pass it as argument for child(userID) method to be reference node in firebase database but it's give a null pointer exception.

as it's supposed to do, i logged in first to get the id of current user

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    firebaseAuth = FirebaseAuth.getInstance();
            firebaseSignIn();
            Log.i("userID", "user id is: " + userID);
            databaseReference = FirebaseDatabase.getInstance().getReference().child("users available");
            userReference = databaseReference.child(userID);  //error line
}

    public void firebaseSignIn() {
            String email = PreferenceManager.getDefaultSharedPreferences(getBaseContext()).getString("email", "");
            String password = PreferenceManager.getDefaultSharedPreferences(getBaseContext()).getString("password", "");
            Log.i(getClass().getName(), "email is: " + email + "and password is: " + password);
            if(email.equals("") || password.equals(""))
                startActivity(new Intent(MapsActivity.this, WelcomeActivity.class));
            else {
                firebaseAuth.signInWithEmailAndPassword(email, password).addOnCompleteListener(new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (task.isSuccessful()) {
                            userID = FirebaseAuth.getInstance().getCurrentUser().getUid();
                            Log.i("userID", "user id is: " + userID);
                            Toast.makeText(MapsActivity.this, R.string.signed_in, Toast.LENGTH_LONG).show();
                        }
                    }
                }).addOnFailureListener(new OnFailureListener() {
                    @Override
                    public void onFailure(@NonNull Exception e) {
                        Toast.makeText(MapsActivity.this, e.getMessage(), Toast.LENGTH_LONG).show();
                    }
                });
            }
        }

this is the error i got.

Caused by: java.lang.NullPointerException: Can't pass null for argument 'pathString' in child()
        at com.google.firebase.database.DatabaseReference.child(com.google.firebase:firebase-database@@17.0.0:101)
        at com.example.findnurse.MapsActivity.onCreate(MapsActivity.java:116)
        at android.app.Activity.performCreate(Activity.java:7340)
        at android.app.Activity.performCreate(Activity.java:7331)