bzr viz stacktraces if you press refresh while it's loading

[Geoff Bache]

Steps to reproduce:

1) Find a reasonably large repository
2) Run "bzr viz" (or press "Log" from olive)
3) Before it's finished loading, press Refresh on the toolbar.

This results in

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/bzrlib/plugins/gtk/branchview/treeview.py", line 161, in do_get_property
    return self.model.get_value(self.model.get_iter(self.path),
AttributeError: 'TreeView' object has no attribute 'model'
/usr/lib/python2.5/site-packages/bzrlib/ui/__init__.py:91: UserWarning: ProgressTask(5/5, msg=None) is not the active task ProgressTask(5/5, msg=None)
  % (task, self._task_stack[-1]))

being written to the console and the progress bar coming to an end before completion, saying "Prettifying graph".

I've had various other errors from pressing things while it was loading (including a stack where TreeView.get_children appeared to have returned None) but this was the easiest to reproduce. Perhaps the GUI should be locked while loading is going on?