Add 'format' argument to built-in fields 'created' & 'modified'.

Author: dadish

src/Field/Traits/DatetimeResolverTrait.php

@@ -28,7 +28,8 @@ public function resolve($value, array $args, ResolveInfo $info)
     if (isset($args['format'])) {
       $format = $args['format'];
       $rawValue = $value->$fieldName;
-      if (Utils::fields()->get($fieldName) instanceof FieldtypeDatetime) {
+      $field = Utils::fields()->get($fieldName);
+      if ($field && $field->type instanceof FieldtypeDatetime) {
         $rawValue = $value->getUnformatted($fieldName);
       }
       if ($rawValue) {