Deleting attachments on an AR fails when returning to "To be verified" state from "Verified" state.

[witekdev]

Steps to reproduce

Using the master branch I created an instance with bika.lims test setup data loaded.
I create an AR. I am able to add and delete attachments on the AR without a problem until I reach the Verified State (where I cannot delete attachments by design), so I return to the "To be verified" state and attempt to delete an attachment, which then fails.

Current behavior

Attachment is not deleted. Traceback:

Traceback (innermost last):
  Module ZPublisher.Publish, line 138, in publish
  Module ZPublisher.mapply, line 77, in mapply
  Module ZPublisher.Publish, line 48, in call_object
  Module bika.lims.browser.attachment, line 81, in __call__
  Module bika.lims.browser.attachment, line 100, in action_update
  Module bika.lims.browser.attachment, line 188, in delete_attachment
AttributeError: 'NoneType' object has no attribute 'getAttachment'

Expected behavior

Attachment is deleted