NPE in provisioning groups UI (corner case / race condition)

Why on earth you would be doing this:

Troubleshooting problems with a model import XML file generated from a script.

Steps to reproduce:

1. Create fhosts.xml in OPENNMS_HOME/etc/imports/pending
2. Import fhosts provisioning group in web UI -> Admin -> Manage Provisioning Groups
3. See that import did not go as expected
4. Click "Delete nodes" button for fhosts provisioning group in web UI
5. Delete OPENNMS_HOME/etc/imports/fhosts.xml
6. Get distracted by a colleague
7. Click "Delete Group" button for fhosts provisioning group in web UI

Expected result:

Provisioning groups UI refreshes minus the fhosts group

Actual result:

NPE trace in web UI.

We should catch it and say something nicer, like "No such provisioning group 'fhosts' exists".