Bug: Exception after changing the field id when the save data action has a defined set of fields to save

[frapell]

Steps to reproduce:

1. Create a Form
2. Go to "Actions" -> "Define form actions"
3. Add a "Save Data" action (mailer can be removed)
4. Go to "Settings..." for the new action
5. Under the "Saved fields" choose all fields (This step is important)
6. Click "Save"
7. Go back to the form in "view" mode and enter some test data
8. Go to "Actions" -> "Saved data"

At this point, the data entered in step 7 should be visible and a CSV can be downloaded. So now:

9. Go to "Actions" -> "Define form fields"
10. Remove the "topic" field
11. Go to "Actions" -> "Saved data"

An exception is raised

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 359, in publish_module
  Module ZPublisher.WSGIPublisher, line 262, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module collective.easyform.browser.actions, line 203, in __call__
  Module plone.z3cform.layout, line 63, in __call__
  Module plone.z3cform.layout, line 47, in update
  Module plone.z3cform.crud.crud, line 386, in update
  Module plone.z3cform.crud.crud, line 209, in update
  Module plone.z3cform.crud.crud, line 218, in _update_subforms
  Module z3c.form.form, line 226, in update
  Module plone.z3cform.patch, line 20, in BaseForm_update
  Module z3c.form.form, line 154, in update
  Module z3c.form.form, line 136, in updateWidgets
  Module z3c.form.field, line 227, in update
  Module plone.z3cform.crud.crud, line 134, in fields
  Module collective.easyform.browser.actions, line 144, in update_schema
  Module z3c.form.field, line 154, in select
  Module z3c.form.field, line 154, in <listcomp>
  Module z3c.form.util, line 259, in __getitem__
KeyError: 'topic'

The reason for this is because the Save Data adapter saves the list of fields (From step 5) as a static list, so when a field is removed or its id is changed (By manually editing the XML fields model), the fields.select(*showFields) fails.

This doesn't happen when there's no field selected in the save data adapter, and so all available fields are used.