failure to display diff viewer from inside "qlog" when entered from "qbrowse"

(This bug is the logical continuation of the steps in bug 189233)

Steps to reproduce:
1. "bzr qbrowse"
2. select a file -> "show log..." -> log window opens
3. select a revision -> revision file list (in the bottom right corner) gets displayed
4. double-click on a single of the revision file list:
     a) diff window does not open
     b) stack trace in the console:

Traceback (most recent call last):
  File "C:/Dokumente und Einstellungen/kuno/Anwendungsdaten/bazaar/2.0/plugins\qbzr\log.py", line 458, in show_file_differences
    self.show_diff_window(rev, rev, [unicode(path)])
  File "C:/Dokumente und Einstellungen/kuno/Anwendungsdaten/bazaar/2.0/plugins\qbzr\log.py", line 437, in show_diff_window
    tree, old_tree = self.branch.repository.revision_trees(revs)
  File "bzrlib\repository.pyc", line 1695, in revision_trees
  File "bzrlib\repository.pyc", line 1513, in _iter_inventories
  File "bzrlib\repofmt\pack_repo.pyc", line 2016, in get_inventory_weave
  File "bzrlib\repofmt\pack_repo.pyc", line 1807, in get_weave
  File "bzrlib\repofmt\pack_repo.pyc", line 1332, in ensure_loaded
bzrlib.errors.ObjectNotLocked: KnitPackRepository('file:///F:/projects/.bzr/repository/') is not locked

[bzr 1.3 windows standalone installer, qbzr 0.9.0]