qannotate: crash changing revision (regression)

[Gary van der Merwe]

bzr: ERROR: exceptions.TypeError: QTextCursor.setPosition(int, QTextCursor.MoveMode mode=QTextCursor.MoveAnchor): argument 1 has unexpected type 'NoneType'

Traceback (most recent call last):
  File "/home/garyvdm/qbzr/trunk/lib/annotate.py", line 737, in default_action
    self.parent_annotate_window.set_annotate_revision()
  File "/home/garyvdm/qbzr/trunk/lib/util.py", line 856, in decorate
    run_in_loading_queue(f, *args, **kargs)
  File "/home/garyvdm/qbzr/trunk/lib/util.py", line 868, in run_in_loading_queue
    f(*args, **kargs)
  File "/home/garyvdm/qbzr/trunk/lib/annotate.py", line 594, in set_annotate_revision
    self.annotate(self.tree, self.fileId, self.path)
  File "/home/garyvdm/qbzr/trunk/lib/annotate.py", line 458, in annotate
    self.text_edit.set_positions(new_positions, lines_to_center)
  File "/home/garyvdm/qbzr/trunk/lib/annotate.py", line 214, in set_positions
    new_center_cursor.setPosition(new_center)
TypeError: QTextCursor.setPosition(int, QTextCursor.MoveMode mode=QTextCursor.MoveAnchor): argument 1 has unexpected type 'NoneType'

bzr 2.2b2 on python 2.6.5 (linux2)
arguments: ['/usr/bin/bzr', 'qannotate', '/home/garyvdm/qbzr/trunk/lib/loggraphprovider.py']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_ZA.utf8'
plugins:
  bzrtools /usr/lib/python2.6/dist-packages/bzrlib/plugins/bzrtools [2.1.0]
  explorer /home/garyvdm/.bazaar/plugins/explorer [1.1.0dev]
  extmerge /home/garyvdm/.bazaar/plugins/extmerge [unknown]
  git /home/garyvdm/.bazaar/plugins/git [0.5.0]
  launchpad /usr/lib/python2.6/dist-packages/bzrlib/plugins/launchpad [2.2b2]
  netrc_credential_store /usr/lib/python2.6/dist-packages/bzrlib/plugins/netrc_credential_store [2.2b2]
  news_merge /usr/lib/python2.6/dist-packages/bzrlib/plugins/news_merge [2.2b2]
  qbzr /home/garyvdm/qbzr/trunk [0.19.0dev1]
  search /home/garyvdm/.bazaar/plugins/search [1.7.0dev]
  testrunner /home/garyvdm/.bazaar/plugins/testrunner [unknown]
  upload /home/garyvdm/.bazaar/plugins/upload [1.0.0dev]