exception with single state workflow

```, 'NoneType' object has no attribute '__getitem__'```
i
Mostly likely where it tries to get the workflow state. https://github.com/collective/collective.contentrules.setfield/blob/master/collective/contentrules/setfield/action.py#L189.

Given you can get the workflow state using ```obj.restrictedTraverse('@@plone_context_state').workflow_state()``` in a script, I don't think you need this.

Start in collective/contentrules/setfield/action.py around line 189, where the issue reports workflow-state lookup failing for a single-state workflow. Compare that lookup with the suggested @@plone_context_state workflow_state() access, then verify that processing a single-state workflow no longer raises the reported TypeError.

Written by the indexing model from the issue text.