fix(dropdownfield): empty dropdown

Signed-off-by: Thierry Bugier <tbugier@teclib.com>

Author: btry

inc/fields/dropdownfield.class.php

@@ -235,7 +235,7 @@ public function displayField($canEdit = true) {
                      $itemtype::getTable(),
                      $decodedValues['show_ticket_categories_root']
                   );
-               $dparams_cond_crit[ItilCategory::getTable().'.id'] = $sons;
+               $dparams_cond_crit[$itemtype::getTable().'.id'] = $sons;
                $rootItem = new $itemtype();
                if ($rootItem->getFromDB($decodedValues['show_ticket_categories_root'])) {
                   $baseLevel = $rootItem->fields['level'];