fix behaviour of CalendarType in Controller and add field users to CalendarType

src/Bundle/ChillMainBundle/Form/DataTransformer/IdToEntityDataTransformer.php

@@ -88,6 +88,10 @@ class IdToEntityDataTransformer implements DataTransformerInterface
             return implode(',', $ids);
         }
 
+        if (null === $value) {
+            return '';
+        }
+
         $id = call_user_func($this->getId, $value);
 
         if (null === $id) {