Can't import ui file
I'm trying to import the following ui file:
<?xml version="1.0" encoding="UTF-8"?>
<interface>
<requires lib="gtk" version="4.0"/>
<template class="Window" parent="GtkApplicationWindow">
<property name="can-focus">0</property>
<property name="default-width">600</property>
<property name="default-height">400</property>
<child>
<object class="GtkLabel"id="label">
<property name="can-focus">0</property>
<property name="label" translatable="yes">Soy una aplicación wonika unu</property>
</object>
</child>
<child type="titlebar">
<object class="GtkHeaderBar">
<property name="can-focus">0</property>
<property name="title-widget">
<object class="GtkLabel">
<property name="label" translatable="yes">Holis uwu</property>
<property name="single-line-mode">1</property>
<property name="ellipsize">end</property>
<property name="width-chars">5</property>
<style>
<class name="title"/>
</style>
</object>
</property>
<child>
<object class="GtkButton" id="print_button">
<property name="label" translatable="yes">Click!</property>
<property name="receives-default">1</property>
<signal name="clicked" handler="print_hello" object="Window" after="no" swapped="no"/>
</object>
</child>
</object>
</child>
</template>
</interface>
It works when I compile my project, yet when I try to import it in Cambalache, I get the following error:
What did I do wrong?