fix issue in superevent template when the superevent doesn't have a preferred_event

gracedb/superevents/templates/superevent.html

@@ -37,7 +37,11 @@ TBD:
             {% endfor %}
         </td>
 
-        <td><a href="{% url "view" preferred_event.graceid %}">{{ preferred_event.graceid }}</a></td>
+        <td>
+            {% if preferred_event %}
+            <a href="{% url "view" preferred_event.graceid %}">{{ preferred_event.graceid }}</a>
+            {% endif %}
+        </td>
         <td>
             <div>
             {% for graceid in internal_events %}
@@ -59,6 +63,7 @@ TBD:
 <br />
 <br />
 
+{% if preferred_event %}
 {% block basic_info %}
 <h2>Preferred Event Info</h2>
 
@@ -105,6 +110,8 @@ TBD:
     </tr>
 </table>
 {% endblock %}
+{% endif %}
+
 </div>
 
 <div class="content-area">

gracedb/superevents/views.py

@@ -27,7 +27,10 @@ def webview(request, superevent_id):
     context['preferred_event'] = superevent.preferred_event
 
     # Display far
-    display_far = superevent.preferred_event.far
+    if superevent.preferred_event is not None:
+        display_far = superevent.preferred_event.far
+    else:
+        display_far = None
     far_is_upper_limit = False
     if display_far and is_external(request.user):
         if display_far < settings.VOEVENT_FAR_FLOOR: