[6.1RC1][Windows] adding custom filters fails on some search views

On windows 7 with the RC1 gtk client I can't add custom filters in some search views, for example in the Marketing Campaigns's "Campaign Follow-up" list view (marketing_campaign module).
Clicking on the [+] icon does not produce any effect, and the error log reveals the following traceback:

Traceback (most recent call last):
  File "widget_search\custom_filter.pyo", line 103, in on_fields_combo_changed
AttributeError: 'gtk.ComboBox' object has no attribute 'get_visible'
Traceback (most recent call last):
  File "widget\view\screen_container.pyo", line 112, in add_custom_filter
  File "widget_search\form.pyo", line 422, in add_custom
  File "widget_search\form.pyo", line 410, in process
  File "widget_search\form.pyo", line 411, in process
AttributeError: 'gtk.Entry' object has no attribute 'get_visible'

Most other search views I tried seemed to work, so it is perhaps specific to certain types of fields.
I couldn't reproduce this on Ubuntu.

All the python libs and versions are those of the RC1 installer as can be downloaded on http://www.openerp.com/downloads