empty branch causes exception for bzr viz

  affects bzr-gtk

Running bzr viz on an empty branch causes the following exception to be
raised:

ganieda:/data/tmp/xp% bzr.dev viz
Traceback (most recent call last):
  File "/home/jelmer/.bazaar/dev-plugins/gtk/branchview/treeview.py",
line 385, in _on_revision_activated
    self.show_diff(revision_id, parent_id)
  File "/home/jelmer/.bazaar/dev-plugins/gtk/branchview/treeview.py",
line 263, in show_diff
    revid = self.get_revision().revision_id
AttributeError: 'NoneType' object has no attribute 'revision_id'

--
Jelmer Vernooij <email address hidden> - http://samba.org/~jelmer/
Jabber: <email address hidden>