When we create a Extension object with default constructor and call equals or hashCode method it will throw NullPointerException as extensionId != null check is missing in these two implementation.

Example:
        Extension ext = new Extension();
        ext.hashCode() - will throw NPE
        Extension ext1 = new Extension()
        ext.equals(ext1) -> will throw NPE.

Since it is not a public API and creating Extension object with out any OID doesn't make sense . It is not very critical.