text: Title: a little miss
  Body: when i forget install this packet ->  apispec
  
  page will back err -> internal server error
  
  just have not other tips, so i search the resource code, 
  in marshmallow_apispec.py
  ![image](https://github.com/flasgger/flasgger/assets/101400891/ab69ea20-f0d6-46fa-8064-b2ef36028709)
  there have some import code, if err ,set Schema = None