The ConfigStorageController::loadMultiple() should use EntityInterface::id() function instead of trying to retrieve a public member. See DatabaseStorageController::loadMultiple() as example.
// Ensure that the returned array is ordered the same as the original
// $ids array if this was passed in and remove any invalid ids.
if ($passed_ids) {
// Remove any invalid ids from the array.
$passed_ids = array_intersect_key($passed_ids, $entities);
foreach ($entities as $entity) {
$passed_ids[$entity->{$this->idKey}] = $entity;
}
$entities = $passed_ids;
}