LoginActivity のビューにアクセスすると NullPointerException が発生するのはなぜですか?

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference

private EditText usernameField, passwordField;
private TextView error;
private ProgressBar progress;

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

    usernameField = (EditText)findViewById(R.id.username);
    passwordField = (EditText)findViewById(R.id.password);
    error = (TextView)findViewById(R.id.error);
    progress = (ProgressBar)findViewById(R.id.progress);
}

new Handler().postDelayed(new Runnable() {
  @Override
  public void run() {
    Intent intent = new Intent(SplashActivity.this, LoginActivity.class);
    startActivity(intent);
    finish();
  }
}, 1500);