#vulnerable code 
                                public Object toMongoObject(final MappedField mf, final MappedClass mc, final Object value) {
        Object mappedValue = value;

        //convert the value to Key (DBRef) if the field is @Reference or type is Key/DBRef, or if the destination class is an @Entity
        if (isAssignable(mf, value) || isEntity(mc)) {
            try {
                if (value instanceof Iterable) {
                    if(isMapped(mf.getSubClass())) {
                      mappedValue = getDBRefs((Iterable) value);
                    } else {
                      mappedValue = value;
                    }
                } else {
                    final Key<?> key = (value instanceof Key) ? (Key<?>) value : getKey(value);
                    if (key == null) {
                        mappedValue = toMongoObject(value, false);
                    } else {
                        mappedValue = keyToRef(key);
                        if (mappedValue == value) {
                            throw new ValidationException("cannot map to @Reference/Key<T>/DBRef field: " + value);
                        }
                    }
                }
            } catch (Exception e) {
                log.error("Error converting value(" + value + ") to reference.", e);
                mappedValue = toMongoObject(value, false);
            }
        } else if (mf != null && mf.hasAnnotation(Serialized.class)) { //serialized
            try {
                mappedValue = Serializer.serialize(value, !mf.getAnnotation(Serialized.class).disableCompression());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        } else if (value instanceof DBObject) {  //pass-through
            mappedValue = value;
        } else {
            mappedValue = toMongoObject(value, EmbeddedMapper.shouldSaveClassName(value, mappedValue, mf));
            if (mappedValue instanceof BasicDBList) {
                final BasicDBList list = (BasicDBList) mappedValue;
                if (list.size() != 0) {
                    if (!EmbeddedMapper.shouldSaveClassName(extractFirstElement(value), list.get(0), mf)) {
                        for (Object o : list) {
                            if (o instanceof DBObject) {
                                ((DBObject) o).removeField(CLASS_NAME_FIELDNAME);
                            }
                        }
                    }
                }
            } else if (mappedValue instanceof DBObject && !EmbeddedMapper.shouldSaveClassName(value, mappedValue, mf)) {
                ((DBObject) mappedValue).removeField(CLASS_NAME_FIELDNAME);
            }
        }

        return mappedValue;
    } 
                            #location 8 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public Object toMongoObject(final MappedField mf, final MappedClass mc, final Object value) {
        Object mappedValue = value;

        //convert the value to Key (DBRef) if the field is @Reference or type is Key/DBRef, or if the destination class is an @Entity
        if (isAssignable(mf, value) || isEntity(mc)) {
            try {
                if (value instanceof Iterable) {
                  MappedClass mapped = getMappedClass(mf.getSubClass());
                  if(mapped != null && Key.class.isAssignableFrom(mapped.getClazz())) {
                      mappedValue = getDBRefs((Iterable) value);
                    } else {
                      mappedValue = value;
                    }
                } else {
                    final Key<?> key = (value instanceof Key) ? (Key<?>) value : getKey(value);
                    if (key == null) {
                        mappedValue = toMongoObject(value, false);
                    } else {
                        mappedValue = keyToRef(key);
                        if (mappedValue == value) {
                            throw new ValidationException("cannot map to @Reference/Key<T>/DBRef field: " + value);
                        }
                    }
                }
            } catch (Exception e) {
                log.error("Error converting value(" + value + ") to reference.", e);
                mappedValue = toMongoObject(value, false);
            }
        } else if (mf != null && mf.hasAnnotation(Serialized.class)) { //serialized
            try {
                mappedValue = Serializer.serialize(value, !mf.getAnnotation(Serialized.class).disableCompression());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        } else if (value instanceof DBObject) {  //pass-through
            mappedValue = value;
        } else {
            mappedValue = toMongoObject(value, EmbeddedMapper.shouldSaveClassName(value, mappedValue, mf));
            if (mappedValue instanceof BasicDBList) {
                final BasicDBList list = (BasicDBList) mappedValue;
                if (list.size() != 0) {
                    if (!EmbeddedMapper.shouldSaveClassName(extractFirstElement(value), list.get(0), mf)) {
                        for (Object o : list) {
                            if (o instanceof DBObject) {
                                ((DBObject) o).removeField(CLASS_NAME_FIELDNAME);
                            }
                        }
                    }
                }
            } else if (mappedValue instanceof DBObject && !EmbeddedMapper.shouldSaveClassName(value, mappedValue, mf)) {
                ((DBObject) mappedValue).removeField(CLASS_NAME_FIELDNAME);
            }
        }

        return mappedValue;
    }