Problemas ao remover objeto de Collection e refletir no banco

org.hibernate.exception.ConstraintViolationException: could not delete collection rows: [br.com.redeamazonica.corp.model.Sistema.direitos#2]

at org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:94)

at org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:66)

at org.hibernate.persister.collection.AbstractCollectionPersister.deleteRows(AbstractCollectionPersister.java:1314)

at org.hibernate.action.CollectionUpdateAction.execute(CollectionUpdateAction.java:84)

at org.hibernate.engine.ActionQueue.execute(ActionQueue.java:279)

at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:263)

at org.hibernate.engine.ActionQueue.executeActions(ActionQueue.java:170)

at org.hibernate.event.def.AbstractFlushingEventListener.performExecutions(AbstractFlushingEventListener.java:321)

at org.hibernate.event.def.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:50)

at org.hibernate.impl.SessionImpl.flush(SessionImpl.java:1027)

at org.hibernate.impl.SessionImpl.managedFlush(SessionImpl.java:365)

at org.hibernate.transaction.JDBCTransaction.commit(JDBCTransaction.java:137)

at br.com.redeamazonica.corp.testes.Testes.excluir(Testes.java:158)

at br.com.redeamazonica.corp.testes.Testes.main(Testes.java:38)

Caused by: java.sql.SQLException: Cannot insert the value NULL into column ‘cod_sistema’, table ‘ControleAcesso.ControleAcesso.Direito’; column does not allow nulls. UPDATE fails.

at net.sourceforge.jtds.jdbc.SQLDiagnostic.addDiagnostic(SQLDiagnostic.java:368)

at net.sourceforge.jtds.jdbc.TdsCore.tdsErrorToken(TdsCore.java:2816)

at net.sourceforge.jtds.jdbc.TdsCore.nextToken(TdsCore.java:2254)

at net.sourceforge.jtds.jdbc.TdsCore.getMoreResults(TdsCore.java:631)

at net.sourceforge.jtds.jdbc.JtdsStatement.processResults(JtdsStatement.java:584)

at net.sourceforge.jtds.jdbc.JtdsStatement.executeSQL(JtdsStatement.java:546)

at net.sourceforge.jtds.jdbc.JtdsPreparedStatement.executeUpdate(JtdsPreparedStatement.java:505)

at org.hibernate.jdbc.NonBatchingBatcher.addToBatch(NonBatchingBatcher.java:46)

at org.hibernate.persister.collection.AbstractCollectionPersister.deleteRows(AbstractCollectionPersister.java:1283)

… 11 more

<class name="Sistema">
	<id name="cod_sistema" type="integer">
		<generator class="increment"></generator>
	</id>
		
	<property name="nome" type="string" length="50" not-null="true"></property>
	<property name="nomeBanco" type="string" length="15" not-null="true"></property>
		
	<set name="controles" >
		<key column="cod_sistema"></key>
		<one-to-many class="Controle" />
	</set>
		
	<set name="direitos" lazy="false">
		<key column="cod_sistema"></key>
		<one-to-many class="Direito" />
	</set>				
</class>

<class name="Direito">
	<id name="cod_direito" type="integer">
		<generator class="increment"></generator>
	</id>
		
	<property name="nome" type="string" length="50" not-null="true"></property>
		
	<many-to-one name="sistema" column="cod_sistema" class="Sistema" not-null="true" lazy="false"></many-to-one>
		
	<set name="perfis" table="PerfilDireito">
		<key column="cod_direito"></key>
		<many-to-many column="cod_perfil" class="Perfil"></many-to-many>		
	</set>		
		
	<set name="controles" >
		<key column="cod_direito"></key>
		<one-to-many class="Controle"/>		
	</set>		
</class>

Session session = null;
	Transaction transaction = null;
	
	try
	{
		
		session = HibernateUtil2.getSessionFactory().openSession();

		transaction = session.beginTransaction();			
			
		Sistema sistema = (Sistema) session.get(Sistema.class, 2);

		Direito direitoRemover = null;
			
		for (Direito direito : sistema.getDireitos())
		{
			if (direito.getCod_direito() == 8)
			{					
				direitoRemover = direito;					
				break;
			}
		}
			
		sistema.getDireitos().remove(direitoRemover);

		session.save(sistema);

		transaction.commit();
			
		System.out.println("\nOK!!!");
	}
	catch (Exception e)
	{
		transaction.rollback();
			
		e.printStackTrace();
	}