[Vala] getting started with vala and glade

[Chris Bare <chris bare gmail com>]

I have a very simple vala program that is trying to load a glade ui file.
The window appears briefly, but then the program exits without error.
What am I missing?
Note that the commented out code that does not use glade works as I expect:

using Gtk;

public class Thelma : Gtk.Application {
    protected override void activate () {
/*
        var window = new Gtk.ApplicationWindow (this);
        var label = new Gtk.Label ("Hello GNOME!");
        window.add (label);
        window.set_title ("Welcome to GNOME");
        window.set_default_size (200, 100);
        window.show_all ();
*/

        var builder = new Builder ();
        builder.add_from_file ("test.ui");
        var window = builder.get_object ("mainWindow") as
Gtk.ApplicationWindow;
        window.show_all ();
    }
}

public int main (string[] args) {
    return new Thelma().run (args);
}

test.ui:
<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.18.3 -->
<interface>
  <requires lib="gtk+" version="3.12"/>
  <object class="GtkApplicationWindow" id="mainWindow">
    <property name="can_focus">False</property>
    <child>
      <object class="GtkLabel" id="label1">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="label" translatable="yes">Thelma</property>
      </object>
    </child>
  </object>
</interface>
-- 
Chris Bare