#vulnerable code 
                                private void appendStructuredElements(final StringBuilder buffer, final LogEvent event) {
        final Message message = event.getMessage();
        final boolean isStructured = message instanceof StructuredDataMessage;

        if (!isStructured && (fieldFormatters!= null && fieldFormatters.size() == 0) && !includeMDC) {
            buffer.append("-");
            return;
        }

        final Map<String, StructuredDataElement> sdElements = new HashMap<String, StructuredDataElement>();
        final Map<String, String> contextMap = event.getContextMap();

        if (mdcRequired != null) {
            checkRequired(contextMap);
        }

        if (fieldFormatters != null) {
            for (final Map.Entry<String, FieldFormatter> sdElement: fieldFormatters.entrySet()) {
                final String sdId = sdElement.getKey();
                final StructuredDataElement elem = sdElement.getValue().format(event);
                sdElements.put(sdId, elem);
            }
        }

        if (includeMDC && contextMap.size() > 0) {
            if (sdElements.containsKey(mdcSDID.toString())) {
                final StructuredDataElement union = sdElements.get(mdcSDID.toString());
                union.union(contextMap);
                sdElements.put(mdcSDID.toString(), union);
            } else {
                final StructuredDataElement formattedContextMap = new StructuredDataElement(contextMap, false);
                sdElements.put(mdcSDID.toString(), formattedContextMap);
            }
        }

        if (isStructured) {
            final StructuredDataMessage data = (StructuredDataMessage) message;
            final Map<String, String> map = data.getData();
            final StructuredDataId id = data.getId();

            if (sdElements.containsKey(id.toString())) {
                final StructuredDataElement union = sdElements.get(id.toString());
                union.union(map);
                sdElements.put(id.toString(), union);
            } else {
                final StructuredDataElement formattedData = new StructuredDataElement(map, false);
                sdElements.put(id.toString(), formattedData);
            }
        }

        if (sdElements.size() == 0) {
            buffer.append("-");
            return;
        }

        for (final Map.Entry<String, StructuredDataElement> entry: sdElements.entrySet()) {
            formatStructuredElement(entry.getKey(), mdcPrefix, entry.getValue(), buffer, checker);
        }
    } 
                            #location 43 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private void appendStructuredElements(final StringBuilder buffer, final LogEvent event) {
        final Message message = event.getMessage();
        final boolean isStructured = message instanceof StructuredDataMessage;

        if (!isStructured && (fieldFormatters!= null && fieldFormatters.size() == 0) && !includeMDC) {
            buffer.append("-");
            return;
        }

        final Map<String, StructuredDataElement> sdElements = new HashMap<String, StructuredDataElement>();
        final Map<String, String> contextMap = event.getContextMap();

        if (mdcRequired != null) {
            checkRequired(contextMap);
        }

        if (fieldFormatters != null) {
            for (final Map.Entry<String, FieldFormatter> sdElement: fieldFormatters.entrySet()) {
                final String sdId = sdElement.getKey();
                final StructuredDataElement elem = sdElement.getValue().format(event);
                sdElements.put(sdId, elem);
            }
        }

        if (includeMDC && contextMap.size() > 0) {
            if (sdElements.containsKey(mdcSDID.toString())) {
                final StructuredDataElement union = sdElements.get(mdcSDID.toString());
                union.union(contextMap);
                sdElements.put(mdcSDID.toString(), union);
            } else {
                final StructuredDataElement formattedContextMap = new StructuredDataElement(contextMap, false);
                sdElements.put(mdcSDID.toString(), formattedContextMap);
            }
        }

        if (isStructured) {
            final StructuredDataMessage data = (StructuredDataMessage) message;
            final Map<String, String> map = data.getData();
            final StructuredDataId id = data.getId();
            final String sdId = getId(id);

            if (sdElements.containsKey(sdId)) {
                final StructuredDataElement union = sdElements.get(id.toString());
                union.union(map);
                sdElements.put(sdId, union);
            } else {
                final StructuredDataElement formattedData = new StructuredDataElement(map, false);
                sdElements.put(sdId, formattedData);
            }
        }

        if (sdElements.size() == 0) {
            buffer.append("-");
            return;
        }

        for (final Map.Entry<String, StructuredDataElement> entry: sdElements.entrySet()) {
            formatStructuredElement(entry.getKey(), mdcPrefix, entry.getValue(), buffer, checker);
        }
    }