Return None, None in parent_and_container() instead of crashing

Still log the information, just in case...

cubicweb_seda/entities/__init__.py

@@ -42,7 +42,10 @@ def parent_and_container(entity):
         parent = entity.cw_adapt_to('IContained').parent
         if container is None:
             # entity may be both container and contained, and in this case is a container
-            assert entity.cw_adapt_to('IContainer'), entity
+            if entity.cw_adapt_to('IContainer') is None:
+                entity.debug('cannot adapt %s as IContainer in parent_and_container()',
+                             entity)
+                return None, None
             container = entity
     else:
         req = entity._cw