This was originally reported by Guillaume Smet on the development list as a regression in 4.1.7 when using Hibernate Search 4.1.1

Search was now throwing an exception during transaction commit in certain cases that didn't happen under 4.1.6.

We have the following stack trace when saving an object:
Caused by: org.hibernate.HibernateException: Error while indexing in Hibernate Search (before transaction completion)
at org.hibernate.search.backend.impl.EventSourceTransactionContext$DelegateToSynchronizationOnBeforeTx.doBeforeTransactionCompletion(EventSourceTransactionContext.java:186)
at org.hibernate.engine.spi.ActionQueue$BeforeTransactionCompletionProcessQueue.beforeTransactionCompletion(ActionQueue.java:662)
at org.hibernate.engine.spi.ActionQueue.beforeTransactionCompletion(ActionQueue.java:307)
at org.hibernate.internal.SessionImpl.beforeTransactionCompletion(SessionImpl.java:607)
at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.beforeTransactionCommit(JdbcTransaction.java:105)
at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:175)
at org.hibernate.ejb.TransactionImpl.commit(TransactionImpl.java:75)
at org.springframework.orm.jpa.JpaTransactionManager.doCommit(JpaTransactionManager.java:512)
... 47 more
Caused by: java.lang.NullPointerException
at org.hibernate.engine.internal.StatefulPersistenceContext.getLoadedCollectionOwnerOrNull(StatefulPersistenceContext.java:851)
at org.hibernate.event.spi.AbstractCollectionEvent.getLoadedOwnerOrNull(AbstractCollectionEvent.java:75)
at org.hibernate.event.spi.InitializeCollectionEvent.<init>(InitializeCollectionEvent.java:36)
at org.hibernate.internal.SessionImpl.initializeCollection(SessionImpl.java:1799)
at org.hibernate.collection.internal.AbstractPersistentCollection$4.doWork(AbstractPersistentCollection.java:524)
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:212)
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:520)
at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:125)
at org.hibernate.collection.internal.PersistentBag.iterator(PersistentBag.java:266)
at org.hibernate.search.engine.spi.AbstractDocumentBuilder.appendContainedInWorkForInstance(AbstractDocumentBuilder.java:296)
at org.hibernate.search.engine.impl.WorkPlan$PerEntityWork.processContainedIn(WorkPlan.java:525)
at org.hibernate.search.engine.impl.WorkPlan$PerClassWork.processContainedInAndPrepareExecution(WorkPlan.java:297)
at org.hibernate.search.engine.impl.WorkPlan.processContainedInAndPrepareExecution(WorkPlan.java:144)
at org.hibernate.search.backend.impl.WorkQueue.prepareWorkPlan(WorkQueue.java:135)
at org.hibernate.search.backend.impl.BatchedQueueingProcessor.prepareWorks(BatchedQueueingProcessor.java:71)
at org.hibernate.search.backend.impl.PostTransactionWorkQueueSynchronization.beforeCompletion(PostTransactionWorkQueueSynchronization.java:86)
at org.hibernate.search.backend.impl.EventSourceTransactionContext$DelegateToSynchronizationOnBeforeTx.doBeforeTransactionCompletion(EventSourceTransactionContext.java:183)
... 54 more

Sanne pointed out that the Hibernate Search tests demonstrated the issue, specifically

org.hibernate.search.test.embedded.EmbeddedTest.testEmbeddedIndexing()

It looked like it was based around some changes to the AbstractPersistentCollection in relation to HHH-7457 and I've created a test (and proposed fix) in order to get back to the previous behaviour.