In my case this exception sometimes happens while trying to commit changes on modified entity.

Stack trace from my app:

java.lang.NullPointerException
     org.hibernate.event.internal.AbstractFlushingEventListener.prepareEntityFlushes(AbstractFlushingEventListener.java:148)
     org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:88)
     org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:51)
     org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1213)
     org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:402)
     org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.beforeTransactionCommit(JdbcTransaction.java:101)
     org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:175)
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