I'm trying to learn GUI programming using python2 and GTKBuilder, but I get a segmentation fault when I run the code. This is my file, created in Glade as a GTKBuilder file:

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <!-- interface-requires gtk+ 3.0 -->
  <object class="GtkWindow" id="mainWindow">
    <property name="can_focus">False</property>
    <child>
      <object class="GtkBox" id="box1">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="orientation">vertical</property>
        <child>
          <object class="GtkBox" id="box2">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="halign">start</property>
            <property name="margin_left">146</property>
            <property name="margin_right">276</property>
            <child>
              <object class="GtkLabel" id="label1">
                <property name="visible">True</property>
                <property name="can_focus">False</property>
                <property name="label" translatable="yes">label</property>
              </object>
              <packing>
                <property name="expand">True</property>
                <property name="fill">False</property>
                <property name="position">0</property>
              </packing>
            </child>
            <child>
              <object class="GtkEntry" id="entryName">
                <property name="visible">True</property>
                <property name="can_focus">True</property>
                <property name="margin_bottom">4</property>
                <property name="hexpand">True</property>
                <property name="vexpand">True</property>
                <property name="invisible_char">?</property>
                <property name="placeholder_text">Please enter your name here...</property>
              </object>
              <packing>
                <property name="expand">True</property>
                <property name="fill">True</property>
                <property name="position">1</property>
              </packing>
            </child>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">0</property>
          </packing>
        </child>
        <child>
          <object class="GtkButton" id="buttonWriteNameToFile">
            <property name="label" translatable="yes">button</property>
            <property name="use_action_appearance">False</property>
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="receives_default">True</property>
            <property name="use_action_appearance">False</property>
            <signal name="clicked" handler="buttonWriteNameToFile_clicked" swapped="no"/>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">1</property>
          </packing>
        </child>
        <child>
          <placeholder/>
        </child>
        <child>
          <placeholder/>
        </child>
      </object>
    </child>
  </object>
</interface>

My python code, based on this question, is this:

#!/usr/bin/env python

import gtk

class NameApp:
    def __init__(self):
        filename = "project.glade"
        builder = gtk.Builder()
        builder.add_from_file(filename)
        builder.connect_signals(self)
        builder.get_object("mainWindow").show_all() 

    def buttonWriteNameToFile_clicked(self, widget):
        print("File write code...")

if __name__ == "__main__":
    app = NameApp()
    gtk.main()

Running the file with python2 yields this error:

name.py:9: Warning: cannot create instance of abstract (non-instantiatable) type `GtkBox'
  builder.add_from_file(filename)
./geany_run_script.sh: line 5: 14897 Segmentation fault      python2 "name.py"

I thought I followed that example as closely as possible, and I don't see any differences outside of the GTKBuilder file. However, the example in the linked question runs successfully on my machine. I don't know if it's relevant, but I'm running Arch Linux x86_64.