problems with Gtk::ComboBoxText and Gtk::Builder

[Stefano Facchini <stefano facchini gmail com>]

Hi,

I have the following code:

  Gtk::ComboBoxText *combo;
  builder->get_widget ("some_combo", combo);
  combo->append ("id", "text");

with the corresponding fragment in .UI file
  <object class="GtkComboBoxText" id="some_combo>
  </object>

When executed, the program gives:

Gtk-CRITICAL **: gtk_combo_box_text_insert: assertion `id_column >= 0'
failed

The ComboBox seems to be created because it appears in the window.
I don't get the same problem if the ComboBoxText is allocated manually 

  Gtk::ComboBoxText *combo;
  combo = new Gtk::ComboBoxText ();
  combo->append ("id", "text");

---> NO ERROR

Am I doing something wrong with Gtk::Builder?