Flasgger failed to load API definition - api

Question

I get this error message when trying to see the Swagger UI with Flasgger:
ERROR in app: Exception on /apispec_1.json [GET]
Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 2446, in wsgi_app
response = self.full_dispatch_request()
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1951, in full_dispatch_request
rv = self.handle_user_exception(e)
File "/usr/local/lib/python3.6/site-packages/flask_cors/extension.py", line 161, in wrapped_function
return cors_after_request(app.make_response(f(*args, **kwargs)))
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1820, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "/usr/local/lib/python3.6/site-packages/flask/_compat.py", line 39, in reraise
raise value
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1949, in full_dispatch_request
rv = self.dispatch_request()
File "/usr/local/lib/python3.6/site-packages/flask/app.py", line 1935, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "/usr/local/lib/python3.6/site-packages/flask/views.py", line 89, in view
return self.dispatch_request(*args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/flask/views.py", line 163, in dispatch_request
return meth(*args, **kwargs)
File "/usr/local/lib/python3.6/site-packages/flasgger/base.py", line 133, in get
return jsonify(self.loader())
File "/usr/local/lib/python3.6/site-packages/flasgger/base.py", line 465, in get_apispecs
prefix_ids=prefix_ids
File "/usr/local/lib/python3.6/site-packages/flasgger/utils.py", line 747, in extract_definitions
if not getattr(item, 'get'):
AttributeError: 'NoneType' object has no attribute 'get'
I tried everything, but I cannot see anything when I access http://localhost:5000/apidocs. I get error message: Fetch error: INTERNAL SERVER ERROR /apispec_1.json