Hi there,

With the flask 2.0.0 upgrade I get the following backtrace in a test suite:

Traceback (most recent call last):
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/flask/app.py", line 2050, in wsgi_app
    ctx.push()
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/flask/ctx.py", line 381, in push
    self.match_request()
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/flask/ctx.py", line 349, in match_request
    result = self.url_adapter.match(return_rule=True)  # type: ignore
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/werkzeug/routing.py", line 1958, in match
    rv = rule.match(path, method)
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/werkzeug/routing.py", line 931, in match
    value = self._converters[name].to_python(value)
  File "xxxxxxxxxxxxxxxxxxxxx/xxxxx/web.py", line 61, in to_python
    if not user.is_authenticated:
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/werkzeug/local.py", line 422, in __get__
    obj = instance._get_current_object()
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/werkzeug/local.py", line 544, in _get_current_object
    return self.__local()  # type: ignore
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/flask_login/utils.py", line 26, in <lambda>
    current_user = LocalProxy(lambda: _get_user())
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/flask_login/utils.py", line 346, in _get_user
    current_app.login_manager._load_user()
  File "xxxxxxxxxxxxxxxxxxxxx/.tox/py39/lib/python3.9/site-packages/flask_login/login_manager.py", line 316, in _load_user
    user_id = session.get('_user_id')
AttributeError: 'NoneType' object has no attribute 'get'

The pseudo code in the falling test, looks like:

@app.route("/test/<installation:installation>")
@flask_login.login_required
def test_route_with_installation(installation):
    return flask.jsonify({....})
with app.test_client(use_cookies=True) as client:
    r = client.get("/test/123")

The session access that failed is done in the installation converter and it looks like:

class InstallationConverter(routing.IntegerConverter):
    def to_python(self, value: str) -> typing.Optional[Installation]:
          user = flask_login.current_user
          if not user.is_authenticated:
              return None
          ...
          return Installation(...)

Environment:

Python version: 3.9.5
Flask version: 2.0.0

If you need more info, just ask :)