STIX 2.1 File observable bug

[chisholm]

To reproduce:

import stix2
stix2.parse_observable({'atime': '2019-06-28T17:53:32Z',
   'contains_refs': ['domain-name--936999fe-d6c1-4838-aa8a-6f3acc5eea1f',
                   'autonomous-system--f3e0df99-a173-48ac-be87-4bad2945b499',
                   'windows-registry-key--077e6e5e-a7d2-4191-acf3-d5a4c83553bf',
                   'mutex--c136aeaf-d520-467f-8fbc-357d5e7b8b3c',
                   'mac-addr--ba103e76-68d8-4efc-b337-a3173f3dc087'],
 'id': 'file--0a41e9b1-640a-4d48-bf3c-57ace6b07c38',
 'mime_type': 'video/ogg',
 'mtime': '2018-11-14T16:20:24Z',
 'name': 'majority.avi',
 'parent_directory_ref': 'directory--f30f0714-4f59-492b-8cd6-8ea08e07bd6f',
 'type': 'file'}, version="2.1")

Exception:

Traceback (most recent call last):
  File "C:\Programming\python\stix\cti-python-stix2\stix2\base.py", line 94, in _check_property
    kwargs[prop_name] = prop.clean(kwargs[prop_name])
  File "C:\Programming\python\stix\cti-python-stix2\stix2\properties.py", line 180, in clean
    valid = self.contained.clean(item)
  File "C:\Programming\python\stix\cti-python-stix2\stix2\properties.py", line 472, in clean
    _validate_id(value, self.spec_version, required_prefix)
UnboundLocalError: local variable 'required_prefix' referenced before assignment

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<stdin>", line 12, in <module>
  File "C:\Programming\python\stix\cti-python-stix2\stix2\core.py", line 172, in parse_observable
    return obj_class(allow_custom=allow_custom, **obj)
  File "C:\Programming\python\stix\cti-python-stix2\stix2\base.py", line 324, in __init__
    super(_Observable, self).__init__(**kwargs)
  File "C:\Programming\python\stix\cti-python-stix2\stix2\base.py", line 177, in __init__
    self._check_property(prop_name, prop_metadata, setting_kwargs)
  File "C:\Programming\python\stix\cti-python-stix2\stix2\base.py", line 356, in _check_property
    super(_Observable, self)._check_property(prop_name, prop, kwargs)
  File "C:\Programming\python\stix\cti-python-stix2\stix2\base.py", line 104, in _check_property
    exc,
  File "<string>", line 3, in raise_from
stix2.exceptions.InvalidValueError: Invalid value for File 'contains_refs': local variable 'required_prefix' referenced before assignment

I think the cause is the way the contains_refs property is defined:

('contains_refs', ListProperty(ReferenceProperty(invalid_types="", spec_version='2.1'))),

invalid_types is assigned the empty string. That means in the ReferenceProperty object, both self.valid_types and self.invalid_types are "falsey": the former is None and the latter is the empty string. That means both branches of the if statement in the clean() method are skipped, which means required_prefix is never assigned a value. That causes the UnboundLocalError when it's used in _validate_id().