GtkBuilder: gtk_widget_show_all: assertion `GTK_IS_WIDGET (widget)' failed

["Mildred Ki'Lya" <ml.mildred593@xxxxxxxxx>]

Hi,
This is my first attempt to use Gtk (version 2.12.11). So I used Gladeto create my GUI and used GtkBuilder and Vala as programming language.
For the moment, I only tried a very simple thing: show the main windowusing GtkBuilder. The code is as follows:
public class Main{
  private static MailBrowser app;
  public static int main (string[] args)  {    Gtk.init (ref args);        app = new MailBrowser();    app.run();
    return 0;  }}


public class MailBrowser {
  private Gtk.Builder builder;  private Gtk.Widget  main_window;
  construct {    try {      builder = new Gtk.Builder();      builder.add_from_file("MailBrowser.ui");      main_window = (Gtk.Widget) builder.get_object("main_window");      main_window.destroy += Gtk.main_quit;    } catch (GLib.Error err) {      var msg = new Gtk.MessageDialog (        null, Gtk.DialogFlags.MODAL, Gtk.MessageType.ERROR, Gtk.ButtonsType.CANCEL,         "Failed to load UI\n" + err.message);      msg.run();      Gtk.main_quit();    }  }    public void show_all(){    main_window.show_all();  }    public void run(){    show_all();    Gtk.main();  }
}



Compilation runs fine, but when I execute the program I get:

(MailBrowser:9851): Gtk-CRITICAL **: gtk_widget_show_all: assertion `GTK_IS_WIDGET (widget)' failed

Then, nothing happens. I suppose the program is waiting for the mainwindow to be closed, but since the above error prevert the main windowto be shown, nothing happens.Ctrl-C quits the program.

I don't think there is a problem in my xml ui description file. itlooks like:
<?xml version="1.0"?><interface>  <requires lib="gtk+" version="2.12"/>  <object class="GtkWindow" id="main_window">    <child>      <!-- .. -->    </child>  </object></interface>

Is there a solution?
Thanks.
Mildred
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