slickswitcher: AttributeError: 'NoneType' object has no attribute 'get_width'

backtrace
-----
switch.py:60:get_num_columns:AttributeError: 'NoneType' object has no attribute
'get_width'

Traceback (most recent call last):
  File "/usr/share/avant-window-navigator/applets/slickswitcher/switch.py",
line 51, in emitted
    func()
  File "/usr/share/avant-window-navigator/applets/slickswitcher/drawing.py",
line 109, in update
    num_columns = self.switch.get_num_columns()
  File "/usr/share/avant-window-navigator/applets/slickswitcher/switch.py",
line 60, in get_num_columns
    workspace_width = current_workspace.get_width()
AttributeError: 'NoneType' object has no attribute 'get_width'

Local variables in innermost frame:
self: <switch.Switch instance at 0x29757e8>
current_workspace: None