When they want to log in my app crashes firebase realtime database [duplicate]

Question

I am using a real-time database (in firebase) and my error is that when trying to enter the account it gives me an error and the application crashes in the case it indicates that the error is that attached the code and one more image clear

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

    final EditText et_correo = findViewById(R.id.et_correo);
    final EditText et_contrasena = findViewById(R.id.et_contrasena);
    final Button btn_ingresar = findViewById(R.id.btn_ingresar);
    final TextView btn_registrate = findViewById(R.id.btn_registrate);

    btn_ingresar.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            final String correotxt = et_correo.getText().toString();
            final String contrasenaTxt = et_contrasena.getText().toString();

            if (correotxt.isEmpty() || contrasenaTxt.isEmpty()) {
                Toast.makeText(LoginActivity.this, "Favor de introducir sus datos", Toast.LENGTH_SHORT).show();
            } else {

                databaseReference.child("nombre").addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot snapshot) {

                       //THIS IS MY ERROR
                        if (snapshot.hasChild(correotxt)) {

                            final String getPassword = snapshot.child(correotxt).child("contrasena").getValue(String.class);
                            if (getPassword.equals(contrasenaTxt)) {
                                Toast.makeText(LoginActivity.this, "Se ha logueado de manera exitosa!", Toast.LENGTH_SHORT).show();
                                startActivity(new Intent(LoginActivity.this, MainActivity.class));
                                finish();
                            }else{
                                Toast.makeText(LoginActivity.this, "Correo o Contraseña incorrectos", Toast.LENGTH_SHORT).show();


                            }

                        }
                    }

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

                    }
                });
                btn_registrate.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {

                        // open RegistroActivity
                        startActivity(new Intent(LoginActivity.this, RegistroActivity.class));
                    }
                });

            }
        }
    });}}

IMG. LOGCAT