title: Grails + Spock: NullPointerException when doing addTo* on an embedded hasMany field
body: <p>I have domain class: </p>
  
  <pre><code>class MarketCommoditiesConfiguration extends MarketConfiguration {
  
      static mapWith = "mongo"
  
      static hasMany = ['commodities': Commodity]
  
      static embedded = ['commodities']
  
      ...
  </code></pre>
  
  <p>Doing <code>save</code> after <code>addToCommodities</code> throws the following exception. </p>
  
  <pre><code>|  java.lang.NullPointerException
      at org.grails.datastore.mapping.model.types.Association.getInverseSide(Association.java:71)
      at org.grails.datastore.mapping.engine.NativeEntryEntityPersister.handleEmbeddedInstance(NativeEntryEntityPersister.java:1254)
      at org.grails.datastore.mapping.engine.NativeEntryEntityPersister.handleEmbeddedInstance(NativeEntryEntityPersister.java:1215)
      at org.grails.datastore.mapping.engine.NativeEntryEntityPersister.handleEmbeddedToMany(NativeEntryEntityPersister.java:1188)
      at org.grails.datastore.mapping.engine.NativeEntryEntityPersister.persistEntity(NativeEntryEntityPersister.java:1077)
      at org.grails.datastore.mapping.engine.NativeEntryEntityPersister.persistEntity(NativeEntryEntityPersister.java:1138)
      at org.grails.datastore.mapping.engine.EntityPersister.persist(EntityPersister.java:160)
      at org.grails.datastore.mapping.core.AbstractSession.persist(AbstractSession.java:522)
      at org.grails.datastore.gorm.GormInstanceApi.doSave(GormInstanceApi.groovy:194)
      at org.grails.datastore.gorm.GormInstanceApi.save_closure5(GormInstanceApi.groovy:162)
      at org.grails.datastore.mapping.core.DatastoreUtils.execute(DatastoreUtils.java:302)
      at org.grails.datastore.gorm.AbstractDatastoreApi.execute(AbstractDatastoreApi.groovy:37)
      at org.grails.datastore.gorm.GormInstanceApi.save(GormInstanceApi.groovy:161)
      at com.apposit.terra.connect.service.MarketService.addCommodityToMarket(MarketService.groovy:248)
      at com.apposit.terra.connect.service.MarketServiceSpec.Test addCommodityToMarket creates a new next month configuration if current month's configuration doesn't exist(MarketServiceSpec.groovy:164)
  </code></pre>