In which file did you encounter the issue?

python-docs-samples/appengine/standard/firebase/firenotes/

Did you change the file? If so, how?

I followed the instruction on the firenotes page to deploy the app to app engine

Describe the issue

When It's trying to save the note, it throws the following error in the browser console:

Failed to load resource: the server responded with a status of 500 ()
my-prototype.appspot.com/:1 XMLHttpRequest cannot load https://backend-dot-my-prototype.appspot.com/notes. Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://my-prototype.appspot.com' is therefore not allowed access. The response had HTTP status code 500.

Any help would be appreciated.