ObjectNotLocked. What I should do?

[Olexandr Byelchenko]

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I have plugin x-bit that help me to control executable bit
in working tree on win32. This plugin operate with inventory.

Today (after upgrade one of my branch to WT4) I've got error:

E:\work\ping-test>bzr x-bit dtr_sync.py
bzr: ERROR: bzrlib.errors.ObjectNotLocked: <WorkingTree4 of E:/work/ping-test> is not locked

Traceback (most recent call last):
  File "bzrlib\commands.pyc", line 650, in run_bzr_catch_errors
  File "bzrlib\commands.pyc", line 612, in run_bzr
  File "bzrlib\commands.pyc", line 304, in run_argv_aliases
  File "E:\Bazaar\plugins.work\x_bit\__init__.py", line 25, in run
  File "bzrlib\workingtree_4.pyc", line 994, in read_working_inventory
  File "bzrlib\workingtree_4.pyc", line 425, in _get_inventory
  File "bzrlib\workingtree_4.pyc", line 830, in _must_be_locked
ObjectNotLocked: <WorkingTree4 of E:/work/ping-test> is not locked

bzr 0.15.0candidate3 on python 2.5.0.final.0 (win32)
arguments: ['D:\\Program Files\\Bazaar\\bzr.EXE', 'x-bit', 'dtr_sync.py']

** please send this report to bazaar at lists.ubuntu.com

The code in question:

    def run(self, files_list, set=False, clear=False):
        from bzrlib.workingtree import WorkingTree

        tree = WorkingTree.open_containing(files_list[0])[0]
        inv = tree.read_working_inventory()

Error raised on last line.
What I do wrong? And what I need to change to make my plugin works
with both WT3 and WT4?

[µ]
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