engine: Fix a possible NPE when the user removes stale policy unit

When the external scheduler is in use and some plugin disappears,
the plugin is still visible (in disabled form) in the Manage
Policy Units dialog. If the user tries to remove it it has to
work correctly and not raise NPE.

Change-Id: I5fa617af3363ec1d71cf8f0a25af43be4b141067
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1065914
Signed-off-by: Martin Sivak <msivak@redhat.com>