Error when initializing modules

I missed this issue when merging in a recent change, to only add services (for the firewall) for modules that have completed setup. Strangely I didn't see this issue at all when testing in vagrant.

When running Plinth out of the source folder, I get this error:

Traceback (most recent call last):
  File "./run", line 21, in <module>
    plinth.__main__.main()
  File "/home/jvalleroy/fbx/plinth/plinth/__main__.py", line 323, in main
    module_loader.load_modules()
  File "/home/jvalleroy/fbx/plinth/plinth/module_loader.py", line 77, in load_modules
    _initialize_module(module_name, modules[module_name])
  File "/home/jvalleroy/fbx/plinth/plinth/module_loader.py", line 135, in _initialize_module
    init()
  File "/home/jvalleroy/fbx/plinth/plinth/modules/privoxy/__init__.py", line 78, in init
    if service.is_enabled():
AttributeError: 'NoneType' object has no attribute 'is_enabled'

It's due to these lines in the init() for privoxy (and other modules):

def init():
    ...
    if service.is_enabled():
        add_shortcut()