Flask(werkzeug) will return None if `application/json` is not specified in the Content-Type`
This is a consequence of this innocent MR: https://github.com/pallets/werkzeug/pull/2355 which makes all our code fail after flask/werkzeug version is >= 2.1.0
The problem is that Flask-Restful is unmaintained, even the creator of this library suggested other libraries to be used instead.