Creating an UI with GtkBuilder that includes a GtkSourceView

main.py

#!/usr/bin/env python3
import gi
gi.require_version('GtkSource', '4')
gi.require_version('Gtk', '3.0') 

from gi.repository import Gtk, GtkSource, GObject
from os.path import abspath, dirname, join

current_dir = abspath(dirname(__file__))

class MyApp(object):

    def __init__(self):
        GObject.type_register(GtkSource.View)
        self.builder = Gtk.Builder()
        self.glade_file = join(current_dir, 'window.ui')
        self.builder.add_from_file(self.glade_file)
        self.window = self.builder.get_object("window")
        self.window.show_all()

if __name__ == '__main__':
    try:
        gui = MyApp()
        Gtk.main()
    except KeyboardInterrupt:
        pass

window.ui

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <!-- interface-requires gtksourceview 3.0 -->
  <!-- interface-requires gtk+ 3.0 -->
  <object class="GtkWindow" id="window">
    <property name="can_focus">False</property>
    <child>
      <object class="GtkScrolledWindow" id="left_scrolledwindow">
        <property name="visible">True</property>
        <property name="can_focus">True</property>
        <property name="shadow_type">in</property>
        <child>
          <object class="GtkSourceView" id="gtksourceview">
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="has_tooltip">True</property>
            <property name="left_margin">2</property>
            <property name="right_margin">2</property>
            <property name="tab_width">4</property>
            <property name="auto_indent">True</property>
            <property name="indent_on_tab">False</property>
          </object>
        </child>
      </object>
      <packing>
        <property name="expand">True</property>
        <property name="fill">True</property>
        <property name="position">0</property>
      </packing>
    </child>
  </object>
</interface>