Cannot run migrate while using an existing database

I am using Django3 and Python3 with the latest version to learn Django.

I want to use an existing sqlite3 database for my django project. But I have problems while I was using the migrate command.

I have done very few steps to my project so far:

1. use the venv, upgrade pip, install django,

2. generate a project,

3. insert an app, and add it into INSTALLED_APPS,

4. In the settings.py, I added the existing database into the DATABASES as follows.

DATABASES = {
    'default': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': os.path.join(BASE_DIR, 'db.sqlite3'),
    },

    'testdb': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': os.path.join(BASE_DIR, 'TestDB.sqlite3'),
    },
}

5. Then I use the method as here shows, to generate the models.py file. Using existing database in Django

The generated models.py looks like: (where I added the max_length=25)

from django.db import models


class Stars(models.Model):
    actorname = models.CharField(db_column='ActorName', blank=True, null=True, max_length=25)
    realname = models.CharField(db_column='RealName', blank=True, null=True, max_length=25)

    class Meta:
        managed = False
        db_table = 'Stars'

6. Then I run python manage.py migrate and/or python manage.py makemigrations, here I see the following errors:

    (venv) PS D:\Workfolder_Web\testProject\backend\project> python .\manage.py migrate
    Traceback (most recent call last):
    File ".\manage.py", line 21, in <module>
        main()
    File ".\manage.py", line 17, in main
        execute_from_command_line(sys.argv)
    File "D:\Workfolder_Web\testProject\backend\venv\lib\site-packages\django\core\management\__init__.py", line 401, in execute_from_command_line 
        utility.execute()
    File "D:\Workfolder_Web\testProject\backend\venv\lib\site-packages\django\core\management\__init__.py", line 377, in execute
        django.setup()
    File "D:\Workfolder_Web\testProject\backend\venv\lib\site-packages\django\__init__.py", line 24, in setup
        apps.populate(settings.INSTALLED_APPS)
    File "D:\Workfolder_Web\testProject\backend\venv\lib\site-packages\django\apps\registry.py", line 114, in populate
        app_config.import_models()
    File "D:\Workfolder_Web\testProject\backend\venv\lib\site-packages\django\apps\config.py", line 211, in import_models
        self.models_module = import_module(models_module_name)
    File "C:\Users\Administrator\AppData\Local\Programs\Python\Python38-32\lib\importlib\__init__.py", line 127, in import_module
        return _bootstrap._gcd_import(name[level:], package, level)
    File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
    File "<frozen importlib._bootstrap>", line 991, in _find_and_load
    File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
    File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
    File "<frozen importlib._bootstrap_external>", line 779, in exec_module
    File "<frozen importlib._bootstrap_external>", line 916, in get_code
    File "<frozen importlib._bootstrap_external>", line 846, in source_to_code
    File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
    ValueError: source code string cannot contain null bytes

7. My database TestDB.sqlite3 is very simple, it looks like:

Question: How could I debug it? I am not using any bootstrap, urls, views or routers in my current codes.

Thanks for the help!