can load old session

tiramisu/config.py

@@ -1206,8 +1206,10 @@ class KernelMixConfig(KernelGroupConfig):
                           storage=None,
                           new=False,
                           ):
+        if storage is None:
+            storage = self._storage
         if not new:
-            if session_id not in [child.impl_getname() for child in self._impl_children]:
+            if session_id not in await list_sessions(storage=storage):
                 raise ConfigError(_('cannot find existing config with session_id to "{}"').format(session_id))
 
         assert type_ in ('config', 'metaconfig', 'mixconfig'), _('unknown type {}').format(type_)