Always use <child> rather than property name="child"

data/interface.ui

@@ -30,10 +30,10 @@
     <child>
       <object class="GtkScrolledWindow">
         <property name="height_request">600</property>
-        <property name="child">
+        <child>
           <object class="GtkViewport">
             <property name="can_focus">0</property>
-            <property name="child">
+            <child>
               <object class="GtkListBox" id="listbox">
                 <property name="can_focus">0</property>
                 <property name="selection_mode">none</property>
@@ -49,9 +49,9 @@
                   </object>
                 </child>
               </object>
-            </property>
+            </child>
           </object>
-        </property>
+        </child>
       </object>
     </child>
     <child type="titlebar">