Fix that ugly thing right before it disappears.

(while solving another goulish git merge)

user/themes/textbook/templates/partials/language-selector.html.twig

@@ -14,7 +14,7 @@
 
     {% set show_language = true %}
 
-    {% if uri.paths(0) == "lesson" %}
+    {% if uri.paths and uri.paths(0) == "lesson" %}
         {% set base_lang_url = base_url_simple ~ grav.language.getLanguageURLPrefix(language) %}
         {% set lang_url = base_lang_url ~ uri.path %}
     {% else %}