CORS issue in ionic 4 while dealing with firebase cloud functions

Pls help me with this issue.

“Access to fetch at ‘https://null-myionicshop-693bc.cloudfunctions.net/subscribeToTopic’ from origin ‘http://localhost:8100’ has been blocked by CORS policy: Response to preflight request doesn’t pass access control check: No ‘Access-Control-Allow-Origin’ header is present on the requested resource. If an opaque response serves your needs, set the request’s mode to ‘no-cors’ to fetch the resource with CORS disabled”