Change button icon in constructor after displaying the window

Question

I want to change the icon of a button through the constructor method. But it's not happening as I wish. See the following code:

demo.glade:

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.18.3 -->
<interface>
  <requires lib="gtk+" version="3.12"/>
  <object class="GtkImage" id="image1">
    <property name="visible">True</property>
    <property name="can_focus">False</property>
    <property name="icon_name">network-transmit-receive</property>
    <property name="icon_size">2</property>
  </object>
  <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="box3">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <child>
          <object class="GtkButtonBox" id="buttonbox1">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="orientation">vertical</property>
            <property name="layout_style">start</property>
            <child>
              <object class="GtkButton" id="button2">
                <property name="label" translatable="yes">button1</property>
                <property name="visible">True</property>
                <property name="can_focus">True</property>
                <property name="receives_default">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="button3">
                <property name="label" translatable="yes">button2</property>
                <property name="visible">True</property>
                <property name="can_focus">True</property>
                <property name="receives_default">True</property>
              </object>
              <packing>
                <property name="expand">True</property>
                <property name="fill">True</property>
                <property name="position">1</property>
              </packing>
            </child>
            <child>
              <object class="GtkButton" id="button4">
                <property name="label" translatable="yes">button2</property>
                <property name="visible">True</property>
                <property name="can_focus">True</property>
                <property name="receives_default">True</property>
              </object>
              <packing>
                <property name="expand">True</property>
                <property name="fill">True</property>
                <property name="position">2</property>
              </packing>
            </child>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">0</property>
          </packing>
        </child>
        <child>
          <placeholder/>
        </child>
      </object>
      <packing>
        <property name="expand">False</property>
        <property name="fill">True</property>
        <property name="position">0</property>
      </packing>
    </child>
    <child>
      <object class="GtkBox" id="box2">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <child>
          <object class="GtkStatusbar" id="statusbar1">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="valign">center</property>
            <property name="margin_start">10</property>
            <property name="margin_end">10</property>
            <property name="hexpand">True</property>
            <property name="orientation">vertical</property>
            <property name="spacing">2</property>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">0</property>
          </packing>
        </child>
        <child>
          <object class="GtkSpinner" id="spinner1">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
            <property name="halign">center</property>
            <property name="valign">center</property>
            <property name="active">True</property>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">1</property>
          </packing>
        </child>
        <child>
          <object class="GtkButton" id="button1">
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="receives_default">True</property>
            <property name="halign">center</property>
            <property name="valign">center</property>
            <property name="image">image1</property>
            <property name="always_show_image">True</property>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">2</property>
          </packing>
        </child>
      </object>
      <packing>
        <property name="expand">False</property>
        <property name="fill">True</property>
        <property name="position">1</property>
      </packing>
    </child>
  </object>
</interface>

demo.py

import gi

gi.require_version('Gtk','3.0')

from gi.repository import Gtk
from others.dbconnections import PgSqlConnection

class MyWind(Gtk.ApplicationWindow):

    def __init__(self, app):
        super().__init__(application=app)
        self.builder = Gtk.Builder.new_from_file('demo.glade')

        self.add(self.builder.get_object('box1'))

        if not PgSQLConnection.test():
            self.builder.get_object('image1').set_from_icon_name('network-error', Gtk.IconSize.SMALL_TOOLBAR)

        self.builder.get_object('spinner1').stop()

The PgSqlConnection.test() is a method that returns True if the connection was successful otherwise it returns False. However, this method takes a few seconds to return. So the window is displayed only after the return of the method. This is not the behavior I want. I want to display the window while waiting for the return of the method test. The component spinner1 is active by default. Depending on the return, the icon of the button1 is changes and spinner1 will be disabled. The spinner1 component is a animation that indicate that something is being processed.