Reset test for article.path before calling getPageByPath() in summary

_layouts/website/summary.html

 {% macro articles(_articles) %}
     {% for article in _articles %}
         <li class="chapter {% if article.path == file.path and not article.anchor %}active{% endif %}" data-level="{{ article.level }}" {% if article.path %}data-path="{{ article.path|resolveFile }}"{% endif %}>
-            {% if getPageByPath(article.path) %}
+            {% if article.path and getPageByPath(article.path) %}
                 <a href="{{ article.path|resolveFile }}{{ article.anchor }}">
             {% elif article.url %}
                 <a target="_blank" href="{{ article.url }}">