Removing a collection in the JI causes the client to crash on next refresh

[zenmonkeykstop]

To reproduce:

• add a source with some messages, files
• view the source conversation in the client
• delete the source in the JI
• click refresh in the client.

Client deletes collection, then crashes with a db-related error:

Running app with home directory: /home/user/test_data_dir
Traceback (most recent call last):
  File "/home/user/securedrop-client/securedrop_client/logic.py", line 424, in update_conversation_view
    self.session.refresh(self.gui.current_source)
  File "/home/user/.local/share/virtualenvs/securedrop-client-g9KXN1tj/lib/python3.5/site-packages/sqlalchemy/orm/session.py", line 1498, in refresh
    self._expire_state(state, attribute_names)
  File "/home/user/.local/share/virtualenvs/securedrop-client-g9KXN1tj/lib/python3.5/site-packages/sqlalchemy/orm/session.py", line 1600, in _expire_state
    self._validate_persistent(state)
  File "/home/user/.local/share/virtualenvs/securedrop-client-g9KXN1tj/lib/python3.5/site-packages/sqlalchemy/orm/session.py", line 2047, in _validate_persistent
    state_str(state))
sqlalchemy.exc.InvalidRequestError: Instance '<Source at 0x3acaf98fda0>' is not persistent within this Session