Fix for crash, described in bug#19801

Summary:
Interactor from (*it) can be deleted during HandleEvent(), so added additional check.
Otherwise there will be a crash, described in http://bugs.mitk.org/show_bug.cgi?id=19801

Signed-off-by: Alexander Kuznetsov <alexander.ev.kuznetsov@gmail.com>

Test Plan: Tested manually and is basically just an added isNull check, any different changes in behavior should hopefully be caught by the mitkDispatcherTest

Reviewers: kislinsk

Reviewed By: kislinsk

Subscribers: kuznetsov

Maniphest Tasks: T19801

Differential Revision: https://phabricator.mitk.org/D7