fix use after free

docker/controller.py

@@ -1211,6 +1211,7 @@ class PushManager(Manager):
             reset()
 
             with ses.begin():
+                version = ses.query(WebappVersion).filter_by(id=version_id).one()
                 prev = ses.query(WebappVersion).filter_by(
                         webapp_id = version.webapp_id,
                         number    = version.number,