qbrowse diff crashes if branch deleted

[Simon Kersey]

The following exception is produced by qbrowse if you open qbrowse on a working tree and then delete the working tree using operating system commands.

bzr: ERROR: pywintypes.error: (3, 'CreateFileW', 'The system cannot find the path specified.')

Traceback (most recent call last):
  File "C:/Program Files/Bazaar/plugins\qbzr\lib\trace.py", line 291, in reports_exception_decorate
  File "C:/Program Files/Bazaar/plugins\qbzr\lib\diffwindow.py", line 234, in initial_load
  File "C:/Program Files/Bazaar/plugins\qbzr\lib\diffwindow.py", line 242, in load_branch_info
  File "C:/Program Files/Bazaar/plugins\qbzr\lib\diff_arg.py", line 162, in get_diff_window_args
  File "C:/Program Files/Bazaar/plugins\qbzr\lib\diff_arg.py", line 156, in load_old_tree
  File "bzrlib\workingtree.pyo", line 404, in basis_tree
  File "bzrlib\decorators.pyo", line 138, in read_locked
  File "bzrlib\workingtree_4.pyo", line 578, in lock_read
  File "bzrlib\dirstate.pyo", line 3046, in lock_read
  File "bzrlib\lock.pyo", line 372, in __init__
  File "bzrlib\lock.pyo", line 352, in _open
error: (3, 'CreateFileW', 'The system cannot find the path specified.')

bzr 2.1.0rc1 on python 2.5.4 (win32)
arguments: ['bzr', 'qbrowse', 'c:\\sjk\\BazaarPluginDev\\bzr-explorer\\fred']
encoding: 'cp1252', fsenc: 'mbcs', lang: None
plugins:
  bzrtools C:\Program Files\Bazaar\plugins\bzrtools [2.1.0b1]
  explorer C:\Program Files\Bazaar\plugins\explorer [1.1.0dev]
  fastimport C:\Program Files\Bazaar\plugins\fastimport [0.9.0dev]
  launchpad C:\Program Files\Bazaar\plugins\launchpad [2.1.0rc1]
  netrc_credential_store C:\Program Files\Bazaar\plugins\netrc_credential_store [2.1.0rc1]
  qbzr C:\Program Files\Bazaar\plugins\qbzr [0.19.0dev1]
  rebase C:\Program Files\Bazaar\plugins\rebase [0.5.5]
  trunk C:\SJK\BazaarPluginDev\bzr-reserved-edit\trunk [0.1.0dev]
  upload C:\Program Files\Bazaar\plugins\upload [1.0.0dev]
  xmloutput C:\Program Files\Bazaar\plugins\xmloutput [0.8.5]

[Gary van der Merwe (garyvdm)]

This can also be reproduced with bzr remove-tree

If the wt no longer exists, we should switch to the last revision.

I think that the trace back above is unrelated. I get this traceback:
bzr: ERROR: [Errno 2] No such file or directory: u'/home/garyvdm/cfs/addr_usage/.bzr/checkout/dirstate'

Traceback (most recent call last):
  File "_dirstate_helpers_pyx.pyx", line 1852, in bzrlib._dirstate_helpers_pyx._loop_one_block