Entendendo a classe GtkWindow

<?php
  // Cria uma nova janela
  $janela = new GtkWindow();
 
  // Adiciona dois botões
  $janela->add(new GtkButton('Botão 1'));
  $janela->add(new GtkButton('Botão 2'));
?>

(estudos.php:2368): Gtk-WARNING **: Attempting to 
add a widget with type GtkButt on to a GtkWindow, 
but as a GtkBin subclass a GtkWindow can only contain
one wid get at a time; it already contains a widget
of type GtkButton