Isotope view breaks when removing a filter from control panel before removing it from page settings

[fulv]

Test setup

1. Add Subject to Available Filters on @@collective-isotope-settings
2. Create a collection or folder and set its display layout to be Isotope View
3. On the @@isotope-config panel for the item created in 2., add Subject to Filtering

Test case

1. Go back to @@collective-isotope-settings and delete the row for Subject
2. Browse to object created in step 2. above.

2016-01-29 00:32:50 ERROR Zope.SiteErrorLog 1454027570.480.748579147188 http://localhost:8080/Plone/hikes-events/hikes-events/isotope-view
Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module Products.PDBDebugMode.runcall, line 70, in pdb_runcall
  Module ZPublisher.Publish, line 48, in call_object
  Module Products.Five.browser.metaconfigure, line 485, in __call__
  Module Products.Five.browser.pagetemplatefile, line 125, in __call__
  Module Products.Five.browser.pagetemplatefile, line 59, in __call__
  Module zope.pagetemplate.pagetemplate, line 132, in pt_render
  Module five.pt.engine, line 93, in __call__
  Module z3c.pt.pagetemplate, line 163, in render
  Module chameleon.zpt.template, line 258, in render
  Module chameleon.template, line 191, in render
  Module chameleon.template, line 171, in render
  Module 8ccc71b14e50d91ffa8445bfb0085fd6.py, line 1531, in render
  Module 0a95155a6e792c138fae4b60abe9bc28.py, line 1223, in render_master
  Module 0a95155a6e792c138fae4b60abe9bc28.py, line 420, in render_content
  Module 8ccc71b14e50d91ffa8445bfb0085fd6.py, line 1519, in __fill_content_core
  Module 8ccc71b14e50d91ffa8445bfb0085fd6.py, line 1400, in render_content_core
  Module 8ccc71b14e50d91ffa8445bfb0085fd6.py, line 135, in render_listing
  Module five.pt.expressions, line 154, in __call__
  Module five.pt.expressions, line 126, in traverse
  Module zope.traversing.adapters, line 136, in traversePathElement
   - __traceback_info__: (<Products.Five.metaclass.SimpleViewClass from /vagrant/buildout/zeoserver/src/collective.isotope/src/collective/isotope/browser/templates/isotope.pt object at 0xae31866c>, 'filters')
  Module zope.traversing.adapters, line 42, in traverse
   - __traceback_info__: (<Products.Five.metaclass.SimpleViewClass from /vagrant/buildout/zeoserver/src/collective.isotope/src/collective/isotope/browser/templates/isotope.pt object at 0xae31866c>, 'filters', ())
  Module zope.cachedescriptors.property, line 71, in __get__
  Module collective.isotope.browser.views, line 154, in filters
  Module collective.isotope.browser.views, line 188, in _get_filter_label
  Module collective.isotope.browser.views, line 199, in _get_column_label
  Module zope.schema.vocabulary, line 140, in getTerm
LookupError: Subject