Error generating python code for server

Description

Hello everybody I have faced the problem while generating python Flask server from Swagger Editor online.

I have downloaded this project and opened it in Visual Studio Code .
Here are all mine requirements:

flask==1.1.1
werkzeug==0.16.0
connexion==2.6.0

That are installed into virtual inviroments folder. I do not understand what is inccorrect, because I have followed all instaructions given in README file.

(.venv) C:\Users\Fedor\Documents\python\projects\python-flask-server-generated\python-flask-server>python -m swagger_server
Traceback (most recent call last):
  File "C:\Users\Fedor\AppData\Local\Programs\Python\Python38\lib\runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "C:\Users\Fedor\AppData\Local\Programs\Python\Python38\lib\runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "C:\Users\Fedor\Documents\python\projects\python-flask-server-generated\python-flask-server\swagger_server\__main__.py", line 5, in <module>
    from swagger_server import encoder
  File "C:\Users\Fedor\Documents\python\projects\python-flask-server-generated\python-flask-server\swagger_server\encoder.py", line 4, in <module>
    from swagger_server.models.base_model_ import Model
  File "C:\Users\Fedor\Documents\python\projects\python-flask-server-generated\python-flask-server\swagger_server\models\__init__.py", line 9, in <module>
    from swagger_server.models.pet import Pet
  File "C:\Users\Fedor\Documents\python\projects\python-flask-server-generated\python-flask-server\swagger_server\models\pet.py", line 12, in <module>
    class Pet(Model):
  File "C:\Users\Fedor\Documents\python\projects\python-flask-server-generated\python-flask-server\swagger_server\models\pet.py", line 18, in Pet
    def __init__(self, id: int=None, category: Category=None, name: str=None, photo_urls: List[str]=None, tags: List[Tag]=None, status: str=None):  # noqa: E501

NameError: name 'Category' is not defined

The project structure is given with a screenschot

What have I done not correct? Why do I see this traceback?
Thanks in advance!

Start by following the README in the downloaded Python Flask server project and reproduce the traceback with python -m swagger_server. Inspect swagger_server/models/pet.py, swagger_server/models/__init__.py, and the generated model files around the undefined Category reference. Done means the generated server starts without this NameError.