core: Fix FindBugs issue in EventNotificationMethod

Fixes FindBugs issue in EventNotificationMethod introduced in commit
53074bc0614da9d4ef18b83bd8ba12e463c3ec09

Change-Id: I3dafb358db93280d58e40ca1056230ae1dba1767
Signed-off-by: Martin Perina <mperina@redhat.com>