Fix bug in accompanying period export element

Tests/Export/Filter/AccompanyingPeriodFilterTest.php

@@ -89,7 +89,13 @@ class AccompanyingPeriodFilterTest extends AbstractFilterTest
                 ->from('ChillPersonBundle:Person', 'person')
                 ->join('person.accompanyingPeriods', 'accompanying_period')
                 // add a dummy where clause
-                ->where('person.firstname IS NOT NULL')
+                ->where('person.firstname IS NOT NULL'),
+            $em->createQueryBuilder()
+                ->select('activity.date AS date')
+                ->select('activity.attendee as attendee')
+                ->from("ChillActivityBundle:Activity", 'activity')
+                ->join('activity.person', 'person')
+                ->join('person.center', 'center')
         );
     }
 }