core: fixing NPE on ExecutionHandler

Fix a NPE issue found by FindBugs

http://jenkins.ovirt.org/job/ovirt_engine_find_bugs/4646/findbugsResult/module.-609324659/package.1362796474/source.6451/#288

Change-Id: I35034cd331476b6c42a266504979be848ae63c93
Signed-off-by: Eli Mesika <emesika@redhat.com>