#vulnerable code 
                                private void mapOneToMany(Collection<Edge> oneToManyRelationships) {

        EntityCollector entityCollector = new EntityCollector();
        List<MappedRelationship> relationshipsToRegister = new ArrayList<>();
        Set<Edge> registeredEdges = new HashSet<>();

        // first, build the full set of related entities of each type and direction for each source entity in the relationship
        for (Edge edge : oneToManyRelationships) {

            Object instance = mappingContext.getNodeEntity(edge.getStartNode());
            Object parameter = mappingContext.getNodeEntity(edge.getEndNode());

            // is this a relationship entity we're trying to map?
            Object relationshipEntity = mappingContext.getRelationshipEntity(edge.getId());
            if (relationshipEntity != null) {
                // establish a relationship between
                FieldInfo outgoingWriter = findIterableWriter(instance, relationshipEntity, edge.getType(), OUTGOING);
                if (outgoingWriter != null) {
                    entityCollector.recordTypeRelationship(edge.getStartNode(), relationshipEntity, edge.getType(), OUTGOING);
                    relationshipsToRegister.add(new MappedRelationship(edge.getStartNode(), edge.getType(), edge.getEndNode(), edge.getId(), instance.getClass(), ClassUtils.getType(outgoingWriter.typeParameterDescriptor())));
                }
                FieldInfo incomingWriter = findIterableWriter(parameter, relationshipEntity, edge.getType(), Relationship.INCOMING);
                if (incomingWriter != null) {
                    entityCollector.recordTypeRelationship(edge.getEndNode(), relationshipEntity, edge.getType(), Relationship.INCOMING);
                    relationshipsToRegister.add(new MappedRelationship(edge.getStartNode(), edge.getType(), edge.getEndNode(), edge.getId(), instance.getClass(), ClassUtils.getType(incomingWriter.typeParameterDescriptor())));
                }
                if (incomingWriter != null || outgoingWriter != null) {
                    registeredEdges.add(edge);
                }
            } else {
                FieldInfo outgoingWriter = findIterableWriter(instance, parameter, edge.getType(), OUTGOING);
                if (outgoingWriter != null) {
                    entityCollector.recordTypeRelationship(edge.getStartNode(), parameter, edge.getType(), OUTGOING);
                    relationshipsToRegister.add(new MappedRelationship(edge.getStartNode(), edge.getType(), edge.getEndNode(), edge.getId(), instance.getClass(), ClassUtils.getType(outgoingWriter.typeParameterDescriptor())));
                }
                FieldInfo incomingWriter = findIterableWriter(parameter, instance, edge.getType(), Relationship.INCOMING);
                if (incomingWriter != null) {
                    entityCollector.recordTypeRelationship(edge.getEndNode(), instance, edge.getType(), Relationship.INCOMING);
                    relationshipsToRegister.add(new MappedRelationship(edge.getStartNode(), edge.getType(), edge.getEndNode(), edge.getId(), instance.getClass(), ClassUtils.getType(incomingWriter.typeParameterDescriptor())));
                }
                if (incomingWriter != null || outgoingWriter != null) {
                    registeredEdges.add(edge);
                }
            }
        }

        // then set the entire collection at the same time for each owning type
        for (Long instanceId : entityCollector.getOwningTypes()) {
            //get all relationship types for which we're trying to set collections of instances
            for (String relationshipType : entityCollector.getOwningRelationshipTypes(instanceId)) {
                //for each relationship type, get all the directions for which we're trying to set collections of instances
                for (String relationshipDirection : entityCollector.getRelationshipDirectionsForOwningTypeAndRelationshipType(instanceId, relationshipType)) {
                    //for each direction, get all the entity types for which we're trying to set collections of instances
                    for (Class entityClass : entityCollector.getEntityClassesForOwningTypeAndRelationshipTypeAndRelationshipDirection(instanceId, relationshipType, relationshipDirection)) {
                        Collection<?> entities = entityCollector.getCollectiblesForOwnerAndRelationship(instanceId, relationshipType, relationshipDirection, entityClass);
                        //Class entityType = entityCollector.getCollectibleTypeForOwnerAndRelationship(instanceId, relationshipType, relationshipDirection);
                        mapOneToMany(mappingContext.getNodeEntity(instanceId), entityClass, entities, relationshipType, relationshipDirection);
                    }
                }
            }
        }

        // now register all the relationships we've mapped as iterable types into the mapping context
        for (MappedRelationship mappedRelationship : relationshipsToRegister) {
            mappingContext.addRelationship(mappedRelationship);
        }
        // finally, register anything left over. These will be singleton relationships that
        // were not mapped during one->one mapping, or one->many mapping.
        for (Edge edge : oneToManyRelationships) {
            if (!registeredEdges.contains(edge)) {
                Object source = mappingContext.getNodeEntity(edge.getStartNode());
                Object target = mappingContext.getNodeEntity(edge.getEndNode());
                FieldInfo writer = getRelationalWriter(metadata.classInfo(source), edge.getType(), OUTGOING, target);
                if (writer == null) {
                    writer = getRelationalWriter(metadata.classInfo(target), edge.getType(), INCOMING, source);
                }
                // ensures its tracked in the domain
                if (writer != null) {
                    MappedRelationship mappedRelationship = new MappedRelationship(edge.getStartNode(), edge.getType(), edge.getEndNode(), edge.getId(), source.getClass(), ClassUtils.getType(writer.typeParameterDescriptor()));
                    if (!mappingContext.containsRelationship(mappedRelationship)) {
                        mappingContext.addRelationship(mappedRelationship);
                    }
                }
            }
        }
    } 
                            #location 31 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private void mapOneToMany(Collection<Edge> oneToManyRelationships) {

        EntityCollector entityCollector = new EntityCollector();
        List<MappedRelationship> relationshipsToRegister = new ArrayList<>();

        // first, build the full set of related entities of each type and direction for each source entity in the relationship
        for (Edge edge : oneToManyRelationships) {

            Object instance = mappingContext.getNodeEntity(edge.getStartNode());
            Object parameter = mappingContext.getNodeEntity(edge.getEndNode());

            // is this a relationship entity we're trying to map?
            Object relationshipEntity = mappingContext.getRelationshipEntity(edge.getId());
            if (relationshipEntity != null) {
                // establish a relationship between
                FieldInfo outgoingWriter = findIterableWriter(instance, relationshipEntity, edge.getType(), OUTGOING);
                if (outgoingWriter != null) {
                    entityCollector.recordTypeRelationship(edge.getStartNode(), relationshipEntity, edge.getType(), OUTGOING);
                    relationshipsToRegister.add(new MappedRelationship(edge.getStartNode(), edge.getType(), edge.getEndNode(), edge.getId(), instance.getClass(), ClassUtils.getType(outgoingWriter.typeParameterDescriptor())));
                }
                FieldInfo incomingWriter = findIterableWriter(parameter, relationshipEntity, edge.getType(), INCOMING);
                if (incomingWriter != null) {
                    entityCollector.recordTypeRelationship(edge.getEndNode(), relationshipEntity, edge.getType(), INCOMING);
                    relationshipsToRegister.add(new MappedRelationship(edge.getStartNode(), edge.getType(), edge.getEndNode(), edge.getId(), instance.getClass(), ClassUtils.getType(incomingWriter.typeParameterDescriptor())));
                }
            } else {

                // Use getRelationalWriter instead of findIterableWriter
                // findIterableWriter will return matching iterable even when there is better matching single field
                FieldInfo outgoingWriter = getRelationalWriter(metadata.classInfo(instance), edge.getType(), OUTGOING, parameter);
                if (outgoingWriter != null) {
                    if (!outgoingWriter.forScalar()) {
                        entityCollector.recordTypeRelationship(edge.getStartNode(), parameter, edge.getType(), OUTGOING);
                    } else {
                        outgoingWriter.write(instance, parameter);
                    }
                    MappedRelationship mappedRelationship = new MappedRelationship(edge.getStartNode(), edge.getType(), edge.getEndNode(), edge.getId(), instance.getClass(), ClassUtils.getType(outgoingWriter.typeParameterDescriptor()));
                    relationshipsToRegister.add(mappedRelationship);
                }
                FieldInfo incomingWriter = getRelationalWriter(metadata.classInfo(parameter), edge.getType(), INCOMING, instance);
                if (incomingWriter != null) {
                    if (!incomingWriter.forScalar()) {
                        entityCollector.recordTypeRelationship(edge.getEndNode(), instance, edge.getType(), INCOMING);
                    } else {
                        incomingWriter.write(parameter, instance);
                    }
                    relationshipsToRegister.add(new MappedRelationship(edge.getStartNode(), edge.getType(), edge.getEndNode(), edge.getId(), instance.getClass(), ClassUtils.getType(incomingWriter.typeParameterDescriptor())));
                }
            }
        }

        // then set the entire collection at the same time for each owning type
        for (Long instanceId : entityCollector.getOwningTypes()) {
            //get all relationship types for which we're trying to set collections of instances
            for (String relationshipType : entityCollector.getOwningRelationshipTypes(instanceId)) {
                //for each relationship type, get all the directions for which we're trying to set collections of instances
                for (String relationshipDirection : entityCollector.getRelationshipDirectionsForOwningTypeAndRelationshipType(instanceId, relationshipType)) {
                    //for each direction, get all the entity types for which we're trying to set collections of instances
                    for (Class entityClass : entityCollector.getEntityClassesForOwningTypeAndRelationshipTypeAndRelationshipDirection(instanceId, relationshipType, relationshipDirection)) {
                        Collection<?> entities = entityCollector.getCollectiblesForOwnerAndRelationship(instanceId, relationshipType, relationshipDirection, entityClass);
                        //Class entityType = entityCollector.getCollectibleTypeForOwnerAndRelationship(instanceId, relationshipType, relationshipDirection);
                        mapOneToMany(mappingContext.getNodeEntity(instanceId), entityClass, entities, relationshipType, relationshipDirection);
                    }
                }
            }
        }

        // now register all the relationships we've mapped as iterable types into the mapping context
        for (MappedRelationship mappedRelationship : relationshipsToRegister) {
            mappingContext.addRelationship(mappedRelationship);
        }
    }