fixed bug when deleting an entity that has associated reservations

src/Controller/CalendarController.php

@@ -234,9 +234,13 @@ class CalendarController extends ControllerBase {
       // add in pre/post buffer for setup/takedown (rev 7.x-1.3+)
       //  - if the slot is part of buffer we add "setup" to class
       //  - if we don't have admin rights; we also mark it as booked so no one can book in these slots
+      $preslots = 0;
+      $postslots = 0;
+      if(isset($entities[$rid])) {
         $category = $categories[$entities[$rid]->$category_field->getString()];
         $preslots = $category['prebuffer'] / 30;
         $postslots = $category['postbuffer'] / 30;
+      } 
       $startkey = array_search($reservations[$rid][$time]['time'], $times);
       $endkey = $startkey + $time_slots;
       $k = $startkey - $preslots;