grid filter bums out when there are no columns in a grid

If a record has nothing but a record type and internal desktopcouch variables, a valid grid with no columns or rows can be created. grid_filter doesn't handle this well:

/usr/lib/python2.6/dist-packages/quickly/widgets/grid_filter.py:284: GtkWarning: gtk_tree_path_append_index: assertion `index >= 0' failed
  iter = widget.get_model().get_iter(widget.get_active())
/usr/lib/python2.6/dist-packages/quickly/widgets/grid_filter.py:284: GtkWarning: gtk_tree_model_get_iter: assertion `path->depth > 0' failed
  iter = widget.get_model().get_iter(widget.get_active())
Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/quickly/widgets/grid_filter.py", line 284, in __column_changed
    iter = widget.get_model().get_iter(widget.get_active())