pip 19.2 "pip list -o" raises AttributeError

[hobbsd]

Environment

• pip version: 19.2
• Python version: 3.7.2 64-bit
• OS: Window 10

Description
Running pip list -o raises unhandled AttributeError.

How to Reproduce

1. Upgrade pip with python -m pip install -U pip
2. Then run python -m pip list -o
3. An error occurs.

Output

ERROR: Exception:
Traceback (most recent call last):
  File "C:\Users\username\py_envs\projectname\lib\site-packages\pip\_internal\cli\base_command.py", line 188, in main
    status = self.run(options, args)
  File "C:\Users\username\py_envs\projectname\lib\site-packages\pip\_internal\commands\list.py", line 156, in run
    packages = self.get_outdated(packages, options)
  File "C:\Users\username\py_envs\projectname\lib\site-packages\pip\_internal\commands\list.py", line 164, in get_outdated
    dist for dist in self.iter_packages_latest_infos(packages, options)
  File "C:\Users\username\py_envs\projectname\lib\site-packages\pip\_internal\commands\list.py", line 164, in <listcomp>
    dist for dist in self.iter_packages_latest_infos(packages, options)
  File "C:\Users\username\py_envs\projectname\lib\site-packages\pip\_internal\commands\list.py", line 195, in iter_packages_latest_infos
    best_candidate = evaluator.get_best_candidate(all_candidates)
  File "C:\Users\username\py_envs\projectname\lib\site-packages\pip\_internal\index.py", line 729, in get_best_candidate
    best_candidate = max(candidates, key=self._sort_key)
  File "C:\Users\username\py_envs\projectname\lib\site-packages\pip\_internal\index.py", line 710, in _sort_key
    has_allowed_hash = int(link.is_hash_allowed(self._hashes))
  File "C:\Users\username\py_envs\projectname\lib\site-packages\pip\_internal\models\link.py", line 213, in is_hash_allowed
    return hashes.is_hash_allowed(self.hash_name, hex_digest=self.hash)
AttributeError: 'NoneType' object has no attribute 'is_hash_allowed'

[pradyunsg]

Please see #6772.