Please, edit your topics_table_columns macro like this:
<override_macro name="topics_table_columns" requires="app_namespace">
<n.pin_column/>
<n.topics_column count="[n.app_topic_count/]"/>
<n.star_column/>
<n.replies_column/>
<n.last_post_column has_avatar_control="yes" white_space="nowrap"/>
<n.views_column/>
<n.if.page_node.equals.root_node>
<then><n.subapp_column/></then>
</n.if.page_node.equals.root_node>
</override_macro>