Gtk3 - Python3 - Как отображать метки в добавленном дочернем потоке?

Я новичок в Python Gtk3 и действительно изучаю основы. В этом коде я пытаюсь добавить элемент в данные с помощью диалогового окна, а затем сделать так, чтобы он отображался в блоке потока как новый дочерний элемент блока потока в главном окне. Проблема в том, что дочерний элемент flowbox добавлен, но в нем нет метки. Я не могу понять почему.

Подскажите, пожалуйста, что не так в этом коде?

#!/usr/bin/python3
import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk

# MAIN WINDOW #
class PyApp:
    def __init__(self):
        builder = Gtk.Builder()
        builder.add_from_file("ui.glade")

        dic = {"on_quit" : Gtk.main_quit,"on_add" : self.on_add}
        builder.connect_signals(dic)

        self.obj = builder.get_object

        self.items = self.getItems()

        self.box = self.obj("flowbox")
        if len(self.items) > 0:
            for item in self.items:
                self.box.add(Gtk.Label(item[0])) # newItem[0] for item id

        self.obj('main').show_all()

    def on_add(self, widget):
        """Called when the user clicks on the add button"""
        itemDlg = itemDialog();     
        result,newItem = itemDlg.run()
        if (result == 0):
            print('*** Adding new item in items data ***')
            self.items.append([newItem.id,newItem.name])
            label = Gtk.Label(newItem.id)
            self.box.add(label)


    def getItems(self):
        items = [["id1", "name1"],["id2", "name2"],["id3", "name3"]]
        return items

# ADD DIALOG #
class itemDialog:
    def __init__(self, id = "", name = ""):
        self.dlg = app.obj("add")
        self.item = Item(id,name)

    def run(self):
        self.result = self.dlg.run()

        #get the value of the entry fields
        self.item.id = app.obj('input_id').get_text()
        self.item.name = app.obj('input_name').get_text()

        self.dlg.hide()
        #return the result and the item
        return self.result,self.item

class Item:
    """This class represents all item information"""
    def __init__(self, id = "",name = ""):
        self.id = id
        self.name = name

    def getItem(self):
        return [self.id, self.name] 

if __name__ == "__main__":
    app=PyApp()
    Gtk.main()

И напильник на поляне:

<?xml version = "1.0" encoding = "UTF-8"?>
<!-- Generated with glade 3.20.0 -->
<interface>
  <requires lib = "gtk+" version = "3.20"/>
  <object class = "GtkWindow" id = "main">
    <property name = "can_focus">False</property>
    <property name = "default_width">600</property>
    <property name = "default_height">500</property>
    <signal name = "destroy" handler = "on_quit" swapped = "no"/>
    <child>
      <object class = "GtkBox" id = "box">
    <property name = "visible">True</property>
    <property name = "can_focus">False</property>
    <property name = "orientation">vertical</property>
    <child>
      <object class = "GtkToolbar" id = "toolbar">
        <property name = "visible">True</property>
        <property name = "can_focus">False</property>
        <child>
          <object class = "GtkToolButton" id = "btn_add">
            <property name = "visible">True</property>
            <property name = "can_focus">False</property>
            <property name = "use_underline">True</property>
            <property name = "icon_name">list-add</property>
            <signal name = "clicked" handler = "on_add" swapped = "no"/>
          </object>
          <packing>
            <property name = "expand">False</property>
            <property name = "homogeneous">True</property>
          </packing>
        </child>
        <child>
          <object class = "GtkSeparatorToolItem" id = "sep">
            <property name = "visible">True</property>
            <property name = "can_focus">False</property>
          </object>
          <packing>
            <property name = "expand">False</property>
            <property name = "homogeneous">False</property>
          </packing>
        </child>
        <child>
          <object class = "GtkToolButton" id = "btn_quit">
            <property name = "visible">True</property>
            <property name = "can_focus">False</property>
            <property name = "can_default">True</property>
            <property name = "tooltip_text" translatable = "yes">Quitter</property>
            <property name = "use_underline">True</property>
            <property name = "icon_name">application-exit</property>
            <signal name = "clicked" handler = "on_quit" swapped = "no"/>
          </object>
          <packing>
            <property name = "expand">False</property>
            <property name = "homogeneous">True</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 = "GtkFlowBox" id = "flowbox">
        <property name = "visible">True</property>
        <property name = "can_focus">False</property>
        <property name = "min_children_per_line">3</property>
        <property name = "max_children_per_line">3</property>
      </object>
      <packing>
        <property name = "expand">True</property>
        <property name = "fill">True</property>
        <property name = "position">2</property>
      </packing>
    </child>
      </object>
    </child>
    <child type = "titlebar">
      <placeholder/>
    </child>
  </object>
  <object class = "GtkDialog" id = "add">
    <property name = "can_focus">False</property>
    <property name = "resizable">False</property>
    <property name = "type_hint">dialog</property>
    <property name = "urgency_hint">True</property>
    <property name = "transient_for">main</property>
    <property name = "attached_to">main</property>
    <child internal-child = "vbox">
      <object class = "GtkBox">
    <property name = "can_focus">False</property>
    <property name = "margin_left">2</property>
    <property name = "margin_right">2</property>
    <property name = "margin_top">2</property>
    <property name = "margin_bottom">2</property>
    <property name = "orientation">vertical</property>
    <property name = "spacing">2</property>
    <child internal-child = "action_area">
      <object class = "GtkButtonBox">
        <property name = "can_focus">False</property>
        <property name = "layout_style">end</property>
        <child>
          <object class = "GtkButton" id = "btn-cancel">
            <property name = "label">gtk-undo</property>
            <property name = "visible">True</property>
            <property name = "can_focus">True</property>
            <property name = "receives_default">True</property>
            <property name = "use_stock">True</property>
            <property name = "always_show_image">True</property>
          </object>
          <packing>
            <property name = "expand">True</property>
            <property name = "fill">True</property>
            <property name = "position">0</property>
          </packing>
        </child>
        <child>
          <object class = "GtkButton" id = "btn-vali">
            <property name = "label">gtk-add</property>
            <property name = "visible">True</property>
            <property name = "can_focus">True</property>
            <property name = "receives_default">True</property>
            <property name = "use_stock">True</property>
            <property name = "always_show_image">True</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">False</property>
        <property name = "position">0</property>
      </packing>
    </child>
    <child>
      <object class = "GtkGrid">
        <property name = "visible">True</property>
        <property name = "can_focus">False</property>
        <property name = "margin_top">20</property>
        <property name = "margin_bottom">20</property>
        <child>
          <object class = "GtkLabel" id = "label_id">
            <property name = "visible">True</property>
            <property name = "can_focus">False</property>
            <property name = "halign">start</property>
            <property name = "margin_left">5</property>
            <property name = "margin_right">5</property>
            <property name = "margin_top">5</property>
            <property name = "margin_bottom">5</property>
            <property name = "label" translatable = "yes">Id
</property>
          </object>
          <packing>
            <property name = "left_attach">0</property>
            <property name = "top_attach">0</property>
          </packing>
        </child>
        <child>
          <object class = "GtkEntry" id = "input_id">
            <property name = "visible">True</property>
            <property name = "can_focus">True</property>
            <property name = "margin_left">5</property>
            <property name = "margin_right">5</property>
            <property name = "margin_top">5</property>
            <property name = "margin_bottom">5</property>
          </object>
          <packing>
            <property name = "left_attach">1</property>
            <property name = "top_attach">0</property>
          </packing>
        </child>
        <child>
          <object class = "GtkLabel" id = "label_name">
            <property name = "visible">True</property>
            <property name = "can_focus">False</property>
            <property name = "halign">start</property>
            <property name = "margin_left">5</property>
            <property name = "margin_right">5</property>
            <property name = "margin_top">5</property>
            <property name = "margin_bottom">5</property>
            <property name = "label" translatable = "yes">Name</property>
          </object>
          <packing>
            <property name = "left_attach">0</property>
            <property name = "top_attach">1</property>
          </packing>
        </child>
        <child>
          <object class = "GtkEntry" id = "input_name">
            <property name = "visible">True</property>
            <property name = "can_focus">True</property>
            <property name = "margin_left">5</property>
            <property name = "margin_right">5</property>
            <property name = "margin_top">5</property>
            <property name = "margin_bottom">5</property>
          </object>
          <packing>
            <property name = "left_attach">1</property>
            <property name = "top_attach">1</property>
          </packing>
        </child>
      </object>
      <packing>
        <property name = "expand">False</property>
        <property name = "fill">True</property>
        <property name = "position">1</property>
      </packing>
    </child>
      </object>
    </child>
    <action-widgets>
      <action-widget response = "-6">btn-cancel</action-widget>
      <action-widget response = "0">btn-vali</action-widget>
    </action-widgets>
  </object>
</interface>