Unknown action types should be ignored instead of crashing the dashboard

If a new action type is created and used in a dashboard, instead of simply ignoring the new (and unknown to the GTK client) action type and either not displaying anything in its place or putting some kind of empty placeholder (similar to unknown view types), the GTK client simply refuses to display the dashboard at all and prints the following traceback to stdout:

Traceback (most recent call last):
  File "bin/modules/gui/window/tree.py", line 209, in sig_open
    if not self.sig_action(widget, 'tree_but_open', warning=False):
  File "bin/modules/gui/window/tree.py", line 203, in sig_action
    warning=warning)
  File "bin/modules/action/main.py", line 199, in exec_keyword
    self._exec_action(action, data, context=context)
  File "bin/modules/action/main.py", line 143, in _exec_action
    limit=datas['limit'], auto_refresh=datas['auto_refresh'], auto_search = datas['auto_search'], search_view = datas['search_view'])
  File "bin/modules/gui/window/__init__.py", line 48, in create
    auto_refresh=auto_refresh, auto_search=auto_search, search_view=search_view)
  File "bin/modules/gui/window/form.py", line 75, in __init__
    show_search=True, window=self.window, limit=limit, readonly=bool(auto_refresh), auto_search=auto_search, search_view=search_view)
  File "bin/widget/screen/screen.py", line 128, in __init__
    view = self.add_view_id(view_id, view_type[0], help=help)
  File "bin/widget/screen/screen.py", line 588, in add_view_id
    toolbar=view.get('toolbar', False), submenu=view.get('submenu', False), context=context)
  File "bin/widget/screen/screen.py", line 630, in add_view
    view = parser.parse(self, root_node, self.fields, toolbar=toolbar, submenu=submenu, help=help)
  File "bin/widget/view/widget_parse.py", line 57, in parse
    wid, child, buttons, on_write = widget.parse(screen.resource, node, fields)
  File "bin/widget/view/form_gtk/parser.py", line 537, in parse
    _, widgets, saws, on_write = self.parse(model, node, fields, paned=hp)
  File "bin/widget/view/form_gtk/parser.py", line 556, in parse
    widget, widgets, saws, on_write = self.parse(model, node, fields, paned=paned)
  File "bin/widget/view/form_gtk/parser.py", line 565, in parse
    container.wid_add(widget_act.widget, colspan=int(attrs.get('colspan', 3)), expand=True, fill=True)
AttributeError: 'action' object has no attribute 'widget'

This behavior can be tested using the "client actions" server and addons branches:
https://code.launchpad.net/~openerp-dev/openobject-server/trunk-client-actions-xmo
https://code.launchpad.net/~openerp-dev/openobject-addons/trunk-client-actions-xmo

It creates a "client" action type and adds one such action to the Administration dashboard. Trying to open the administration dashboard in the GTK client results in no visible UI action and the traceback above.