Why is the window not rendering correctly?

I'm trying to develop an application using GNOME Builder and Glade, but it will not render.

Here's the .ui code.

https:// pastebin.com/JbfqC6Mt

The window is supposed to render 3 buttons in a row.

i'm not sure if it's my version of something or what. I need this to work as i am one of the main programmers of this project.

`<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.38.2 -->
<interface>
  <requires lib="gtk" version="4.0"/>
  <requires lib="gtk+" version="3.10"/>
  <template class="EmucratelauncherWindow" parent="GtkApplicationWindow">
    <property name="can-focus">False</property>
    <property name="default-width">600</property>
    <property name="default-height">300</property>
    <child>
      <object class="GtkBox">
        <property name="visible">True</property>
        <property name="can-focus">False</property>
        <property name="spacing">100</property>
        <property name="homogeneous">True</property>
        <child>
          <object class="GtkButton">
            <property name="label" translatable="yes">button</property>
            <property name="visible">True</property>
            <property name="can-focus">True</property>
            <property name="receives-default">True</property>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">0</property>
          </packing>
        </child>
        <child>
          <object class="GtkButton">
            <property name="label" translatable="yes">button</property>
            <property name="visible">True</property>
            <property name="can-focus">True</property>
            <property name="receives-default">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">
            <property name="label" translatable="yes">button</property>
            <property name="visible">True</property>
            <property name="can-focus">True</property>
            <property name="receives-default">True</property>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">2</property>
          </packing>
        </child>
      </object>
    </child>
    <child type="titlebar">
      <object class="GtkHeaderBar" id="header_bar">
        <property name="can-focus">False</property>
        <child type="end">
          <object class="GtkMenuButton">
            <property name="can-focus">False</property>
            <property name="receives-default">False</property>
            <child>
              <placeholder/>
            </child>
          </object>
        </child>
      </object>
    </child>
  </template>
</interface>`