Mongo Spring Reactive - Cannot create a reference to an object with a NULL id

I am learning Mongo Spring Reactive. I trying to build a small app. Github Link: https://github.com/ajith428/GARRSolutions

while trying to save the Document which has DBRef, failing to save. Giving an error Cannot create a reference to an object with a NULL id. Tried Cascade Save learnt from https://www.baeldung.com/cascading-with-dbref-and-lifecycle-events-in-spring-data-mongodb

Still getting the same error. I want to save the documents just like SQL mappings(foreign keys and one-to-many, many-to-one mappings),

Thanks in Advance.