NullPointerException when validating associated domain

### Steps to Reproduce
Using Grails 3.2.8
1.- Create domain class A
```
@Resource(readOnly = false, formats = ['json', 'xml'], uri = "/a")
class A {
B b
}
```
2.- Create domain class B
```
class B {
String field1
String field2
}
```
3.- Make POST call ignoring 1 property from associated class B on purpose
```
POST http://localhost:8080/a
{
"b": {
"field1": "test"
}
}
```
### Expected Behaviour
```
400 Bad Request
{
"message": "Property field2 can't be null"
}
```
### Actual Behaviour
```
500 Internal Server Error
2017-04-25 15:42:43.003 ERROR --- [nio-8080-exec-2] o.g.web.errors.GrailsExceptionResolver : NullPointerException occurred when processing request: [POST] /a
Stacktrace follows:

java.lang.reflect.InvocationTargetException: null
at org.grails.core.DefaultGrailsControllerClass$ReflectionInvoker.invoke(DefaultGrailsControllerClass.java:211)
at org.grails.core.DefaultGrailsControllerClass.invoke(DefaultGrailsControllerClass.java:188)
at org.grails.web.mapping.mvc.UrlMappingsInfoHandlerAdapter.handle(UrlMappingsInfoHandlerAdapter.groovy:90)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
at org.springframework.boot.web.filter.ApplicationContextHeaderFilter.doFilterInternal(ApplicationContextHeaderFilter.java:55)
at org.grails.web.servlet.mvc.GrailsWebRequestFilter.doFilterInternal(GrailsWebRequestFilter.java:77)
at org.grails.web.filters.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:67)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException: null
at org.hibernate.engine.internal.StatefulPersistenceContext.addEntity(StatefulPersistenceContext.java:468)
at org.hibernate.action.internal.AbstractEntityInsertAction.makeEntityManaged(AbstractEntityInsertAction.java:126)
at org.hibernate.engine.spi.ActionQueue.addResolvedEntityInsertAction(ActionQueue.java:279)
at org.hibernate.engine.spi.ActionQueue.addInsertAction(ActionQueue.java:254)
at org.hibernate.engine.spi.ActionQueue.addAction(ActionQueue.java:299)
at org.hibernate.event.internal.AbstractSaveEventListener.addInsertAction(AbstractSaveEventListener.java:318)
at org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:275)
at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:182)
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:113)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:192)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:177)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:97)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:73)
at org.grails.orm.hibernate.support.ClosureEventTriggeringInterceptor.onSaveOrUpdate(ClosureEventTriggeringInterceptor.java:81)
at org.hibernate.internal.SessionImpl.fireSaveOrUpdate(SessionImpl.java:651)
at org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:643)
at org.hibernate.engine.spi.CascadingActions$5.cascade(CascadingActions.java:218)
at org.hibernate.engine.internal.Cascade.cascadeToOne(Cascade.java:391)
at org.hibernate.engine.internal.Cascade.cascadeAssociation(Cascade.java:316)
at org.hibernate.engine.internal.Cascade.cascadeProperty(Cascade.java:155)
at org.hibernate.engine.internal.Cascade.cascade(Cascade.java:104)
at org.hibernate.event.internal.AbstractSaveEventListener.cascadeBeforeSave(AbstractSaveEventListener.java:414)
at org.hibernate.event.internal.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:252)
at org.hibernate.event.internal.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:182)
at org.hibernate.event.internal.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:113)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:192)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:177)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:97)
at org.hibernate.event.internal.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:73)
at org.grails.orm.hibernate.support.ClosureEventTriggeringInterceptor.onSaveOrUpdate(ClosureEventTriggeringInterceptor.java:81)
at org.hibernate.internal.SessionImpl.fireSaveOrUpdate(SessionImpl.java:651)
at org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:643)
at org.hibernate.internal.SessionImpl.saveOrUpdate(SessionImpl.java:638)
at org.grails.orm.hibernate.AbstractHibernateGormInstanceApi$_performSave_closure3.doCall(AbstractHibernateGormInstanceApi.groovy:242)
at org.grails.orm.hibernate.GrailsHibernateTemplate.doExecute(GrailsHibernateTemplate.java:286)
at org.grails.orm.hibernate.GrailsHibernateTemplate.execute(GrailsHibernateTemplate.java:230)
at org.grails.orm.hibernate.GrailsHibernateTemplate.execute(GrailsHibernateTemplate.java:116)
at org.grails.orm.hibernate.AbstractHibernateGormInstanceApi.performSave(AbstractHibernateGormInstanceApi.groovy:241)
at org.grails.orm.hibernate.AbstractHibernateGormInstanceApi.save(AbstractHibernateGormInstanceApi.groovy:158)
at org.grails.datastore.gorm.GormEntity$Trait$Helper.save(GormEntity.groovy:151)
at grails.rest.RestfulController.saveResource(RestfulController.groovy:297)
at grails.rest.RestfulController.$tt__save(RestfulController.groovy:103)
at grails.transaction.GrailsTransactionTemplate$2.doInTransaction(GrailsTransactionTemplate.groovy:96)
at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:133)
at grails.transaction.GrailsTransactionTemplate.execute(GrailsTransactionTemplate.groovy:93)
... 14 common frames omitted
```
### Environment Information

- **Operating System**: Mac OSX Sierra
- **Grails Version:** 3.2.8
- **JDK Version:** 1.8

### Example Application

For some reason, null validation of a missing field on the request body make this fail. If instead, you declare a property Double field3 and in the request body you send e.g. "field3": "something", the correct 400 Bad Request is triggered. Only fails when you omit one field of the sub-domain.