Check user is logged-in for Collapse Read button

tildes/tildes/templates/topic.jinja2

@@ -174,7 +174,8 @@
 
         {# Display the "Collapse read" button if the user is tracking comment visits
            but not automatically collapsing old comments, and there are new comments #}
-        {% if request.user.track_comment_visits
+        {% if request.user
+             and request.user.track_comment_visits
              and not request.user.collapse_old_comments
              and topic.comments_since_last_visit > 0 %}
           <button class="btn btn-sm btn-light" data-js-comment-collapse-read-button>Collapse read</button>