GtkBuilder dialog headerbar

<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <requires lib="gtk+" version="3.12"/>
  <template class="Gjs_MyDialog" parent="GtkDialog">
    <property name="can_focus">False</property>
    <property name="type">popup</property>
    <property name="type_hint">dialog</property>
    <property name="width_request">500</property>
    <child internal-child="vbox">
      <object class="GtkBox" id="contentArea">
        <!-- My content -->
      </object>
    </child>
    <child internal-child="headerbar">
      <object class="GtkHeaderBar" id="headerBar">
        <property name="visible">True</property>
        <property name="can-focus">False</property>
        <property name="show-close-button">False</property>
        <child>
          <object class="GtkButton" id="cancelButton">
            <property name="label" translatable="yes">_Cancel</property>
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="use_underline">True</property>
          </object>
          <packing>
            <property name="pack-type">start</property>
          </packing>
        </child>
        <child>
          <object class="GtkButton" id="okButton">
            <property name="label" translatable="yes">_Ok</property>
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="use_underline">True</property>
          </object>
          <packing>
            <property name="pack-type">end</property>
          </packing>
        </child>
      </object>
    </child>
    <style>
      <class name="my-dialog-css-class"/>
    </style>
  </template>
</interface>