sal-service update-fllow return http status 500

Bug can be seen in this job

https://jenkins.opendaylight.org/releng/view/openflowplugin/job/openflowplugin-csit-verify-flow-services-more-stable-lithium/6/artifact/karaf.log

but it was reproduced also with Lithium RC0

https://nexus.opendaylight.org/content/repositories/automatedweeklyreleases-1057/org/opendaylight/integration/distribution-karaf/0.3.0-Lithium-RC0/distribution-karaf-0.3.0-Lithium-RC0.tar.gz

Steps to reproduce:
use integration suite located in

feature:install odl-openflowplugin-flow-services-ui
run integration suite located in

test/csit/suites/openflowplugin/Flows_Additional_TCs/Stat_Manager_extended/