I attached a bug testcase for easy reproducibility.
Instructions:

• unzip the file
• cd HHH-13413 Open
• mvn spring-boot:run

From another shell execute:

• curl -X POST localhost:8080/write/first

And verify the exception:

2019-07-05 17:51:19.821 ERROR 16627 — [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.orm.jpa.JpaSystemException: null id generated for:class com.example.demo.DetailObject; nested exception is org.hibernate.id.IdentifierGenerationException: null id generated for:class com.example.demo.DetailObject] with root cause

org.hibernate.id.IdentifierGenerationException: null id generated for:class com.example.demo.DetailObject

at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:121) ~[hibernate-core-5.4.3.Final.jar:5.4.3.Final]

at org.hibernate.event.internal.DefaultPersistEventListener.entityIsTransient(DefaultPersistEventListener.java:192) ~[hibernate-core-5.4.3.Final.jar:5.4.3.Final]