ValueError: not enough values to unpack

[jayvdb]

Describe the bug
When invalid args are given to URL field_choices/..., especially a model which doesnt include a ., the logic raises an exception.

This occurs for example when a smoke tester hits a server with lots of invalid requests in order to find bugs, including security problems.

To Reproduce
Steps to reproduce the behavior:

1. Go to `field_choices/foobar/blah'
2. See error

Expected behavior
A 400 error should occur indicating that the client request is invalid.

Additional context

Traceback (most recent call last):
  ...
  File "/usr/local/lib/python3.6/site-packages/django/views/generic/base.py", line 71, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/django/utils/decorators.py", line 45, in _wrapper
    return bound_method(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/django/views/decorators/csrf.py", line 54, in wrapped_view
    return view_func(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/braces/views/_forms.py", line 24, in dispatch
    return super(CsrfExemptMixin, self).dispatch(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/braces/views/_access.py", line 407, in dispatch
    request, *args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/django/views/generic/base.py", line 97, in dispatch
    return handler(request, *args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/advanced_filters/views.py", line 34, in get
    app_label, model_name = model.split('.', 1)
ValueError: not enough values to unpack (expected 2, got 1)