NPE when creating a reference from a Grails domain class to a legacy class

When adding a reference from a Grails domain class to a legacy class mapped with Hibernate a NPE results when reloading Hibernate.

Maybe I've done something wrong in my code, but I cant see what it is and the error messages arent really giving me much of a clue.

The origin of the exception appears to be:-

Caused by: java.lang.NullPointerException
at org.codehaus.groovy.grails.orm.hibernate.cfg.GrailsDomainBinder.getMapping(GrailsDomainBinder.java:1050)
at org.codehaus.groovy.grails.orm.hibernate.cfg.GrailsDomainBinder.bindManyToOne(GrailsDomainBinder.java:1804)
at org.codehaus.groovy.grails.orm.hibernate.cfg.GrailsDomainBinder.createClassProperties(GrailsDomainBinder.java:1480)
at org.codehaus.groovy.grails.orm.hibernate.cfg.GrailsDomainBinder.bindRootPersistentClassCommonValues(GrailsDomainBinder.java:1357)
at org.codehaus.groovy.grails.orm.hibernate.cfg.GrailsDomainBinder.bindRoot(GrailsDomainBinder.java:1118)
at org.codehaus.groovy.grails.orm.hibernate.cfg.GrailsDomainBinder.bindClass(GrailsDomainBinder.java:1002)
at org.codehaus.groovy.grails.orm.hibernate.cfg.DefaultGrailsDomainConfiguration.secondPassCompile(DefaultGrailsDomainConfiguration.java:96)
at org.hibernate.cfg.Configuration.buildMappings(Configuration.java:1148)
at org.springframework.orm.hibernate3.LocalSessionFactoryBean.buildSessionFactory(LocalSessionFactoryBean.java:673)
at org.springframework.orm.hibernate3.AbstractSessionFactoryBean.afterPropertiesSet(AbstractSessionFactoryBean.java:211)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1369)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1335)
... 579 more

The minimal project in the attached file reproduces the issue.

Steps to reproduce:-
1. grails run-app - this will work OK
2. Ensure that the MyGrailsObject table is empty
3. uncomment the reference to MyLegacyObject in grails-app/domain/MyGrailsObject.groovy
4. Save. The running app should pick up the change and you should see the NPE.

Cleaning and restarting the app doesnt help.