[8.0] Export product variant fields leads to error: MissingError: ('MissingError', u'One of the documents you are trying to access has been deleted, please try again after refreshing.')

[lukebranch]

@JayVora-SerpentCS ,

Thanks for your work on this module, it's very useful. We've just run into an error in one of our dev databases that is related to your module.

Steps to reproduce:

1. Install shipping_api_ups and dependant modules.
2. Go to Sales >> Products by Category >> [Select product variants in list view] >> Export a field(s)

a popup modal with the following message is displayed:

MissingError: ('MissingError', u'One of the documents you are trying to access has been deleted, please try again after refreshing.')

After uninstalling the shipping_api_ups module and dependant modules this error is gone.

Here's the section of the traceback from the logs:

2015-07-15 07:47:52,032 3301 ERROR WE-New-Master openerp.addons.web.controllers.main: An exception occured during an http request
Traceback (most recent call last):
  File "/opt/odoo/odoo-server/addons/web/controllers/main.py", line 70, in wrap
    return f(*args, **kwargs)
  File "/opt/odoo/odoo-server/addons/web/controllers/main.py", line 1460, in index
    return self.base(data, token)
  File "/opt/odoo/odoo-server/addons/web/controllers/main.py", line 1441, in base
    import_data = Model.export_data(ids, field_names, self.raw_data, context=context).get('datas',[])
  File "/opt/odoo/odoo-server/openerp/http.py", line 908, in proxy
    result = meth(cr, request.uid, *args, **kw)
  File "/opt/odoo/odoo-server/openerp/api.py", line 241, in wrapper
    return old_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/api.py", line 363, in old_api
    result = method(recs, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/models.py", line 939, in export_data
    return {'datas': self.__export_rows(fields_to_export)}
  File "/opt/odoo/odoo-server/openerp/api.py", line 239, in wrapper
    return new_api(self, *args, **kwargs)
  File "/opt/odoo/odoo-server/openerp/models.py", line 890, in __export_rows
    value = record[name]
  File "/opt/odoo/odoo-server/openerp/models.py", line 5573, in __getitem__
    return self._fields[key].__get__(self, type(self))
  File "/opt/odoo/odoo-server/openerp/fields.py", line 823, in __get__
    return record._cache[self]
  File "/opt/odoo/odoo-server/openerp/models.py", line 5933, in __getitem__
    return value.get() if isinstance(value, SpecialValue) else value
  File "/opt/odoo/odoo-server/openerp/fields.py", line 55, in get
    raise self.exception
MissingError: ('MissingError', u'One of the documents you are trying to access has been deleted, please try again after refreshing.')