#vulnerable code 
                                @Override
    public void doRender(ChartTemplate eleTemplate, ChartMultiSeriesRenderData data, XWPFTemplate template)
            throws Exception {
        if (null == data) return;
        XWPFChart chart = eleTemplate.getChart();
        List<XDDFChartData> chartSeries = chart.getChartSeries();
        // validate combo
        if (chartSeries.size() >= 2) {
            long nullCount = data.getSeriesDatas().stream().filter(d -> null == d.getComboType()).count();
            if (nullCount > 0) throw new RenderException("Combo chart must set comboType field of series!");
        }

        // hack for poi 4.1.1+: repair seriesCount value,
        int totalSeriesCount = chartSeries.stream().mapToInt(XDDFChartData::getSeriesCount).sum();
        Field field = ReflectionUtils.findField(XDDFChart.class, "seriesCount");
        field.setAccessible(true);
        field.set(chart, totalSeriesCount);

        int valueCol = 0;
        List<SeriesRenderData> usedSeriesDatas = new ArrayList<>();
        for (XDDFChartData chartData : chartSeries) {
            int orignSize = chartData.getSeriesCount();
            List<SeriesRenderData> seriesDatas = null;
            if (chartSeries.size() <= 1) {
                // ignore combo type
                seriesDatas = data.getSeriesDatas();
            } else {
                seriesDatas = obtainSeriesData(chartData.getClass(), data.getSeriesDatas());
            }
            usedSeriesDatas.addAll(seriesDatas);
            int seriesSize = seriesDatas.size();

            XDDFDataSource<?> categoriesData = createCategoryDataSource(chart, data.getCategories());
            for (int i = 0; i < seriesSize; i++) {
                XDDFNumericalDataSource<? extends Number> valuesData = createValueDataSource(chart,
                        seriesDatas.get(i).getValues(), valueCol);

                XDDFChartData.Series currentSeries = null;
                if (i < orignSize) {
                    currentSeries = chartData.getSeries(i);
                    currentSeries.replaceData(categoriesData, valuesData);
                } else {
                    // add series, should copy series with style
                    currentSeries = chartData.addSeries(categoriesData, valuesData);
                    processNewSeries(chartData, currentSeries);
                }
                String name = seriesDatas.get(i).getName();
                currentSeries.setTitle(name, chart.setSheetTitle(name, valueCol + VALUE_START_COL));
                valueCol++;
            }
            // clear extra series
            removeExtraSeries(chartData, orignSize, seriesSize);
        }

        XSSFSheet sheet = chart.getWorkbook().getSheetAt(0);
        updateCTTable(sheet, usedSeriesDatas);

        removeExtraSheetCell(sheet, data.getCategories().length, totalSeriesCount, usedSeriesDatas.size());

        for (XDDFChartData chartData : chartSeries) {
            plot(chart, chartData);
        }
        chart.setTitleText(data.getChartTitle());
        chart.setTitleOverlay(false);
    } 
                            #location 16 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Override
    public void doRender(ChartTemplate eleTemplate, ChartMultiSeriesRenderData data, XWPFTemplate template)
            throws Exception {
        XWPFChart chart = eleTemplate.getChart();
        List<XDDFChartData> chartSeries = chart.getChartSeries();
        validate(chartSeries, data);

        int totalSeriesCount = ensureSeriesCount(chart, chartSeries);
        int valueCol = 0;
        List<SeriesRenderData> usedSeriesDatas = new ArrayList<>();
        for (XDDFChartData chartData : chartSeries) {
            int orignSize = chartData.getSeriesCount();
            List<SeriesRenderData> currentSeriesData = null;
            if (chartSeries.size() <= 1) {
                // ignore combo type
                currentSeriesData = data.getSeriesDatas();
            } else {
                currentSeriesData = obtainSeriesData(chartData.getClass(), data.getSeriesDatas());
            }
            usedSeriesDatas.addAll(currentSeriesData);
            int currentSeriesSize = currentSeriesData.size();

            XDDFDataSource<?> categoriesData = createCategoryDataSource(chart, data.getCategories());
            for (int i = 0; i < currentSeriesSize; i++) {
                XDDFNumericalDataSource<? extends Number> valuesData = createValueDataSource(chart,
                        currentSeriesData.get(i).getValues(), valueCol);

                XDDFChartData.Series currentSeries = null;
                if (i < orignSize) {
                    currentSeries = chartData.getSeries(i);
                    currentSeries.replaceData(categoriesData, valuesData);
                } else {
                    // add series, should copy series with style
                    currentSeries = chartData.addSeries(categoriesData, valuesData);
                    processNewSeries(chartData, currentSeries);
                }
                String name = currentSeriesData.get(i).getName();
                currentSeries.setTitle(name, chart.setSheetTitle(name, valueCol + VALUE_START_COL));
                valueCol++;
            }
            // clear extra series
            removeExtraSeries(chartData, orignSize, currentSeriesSize);
        }

        XSSFSheet sheet = chart.getWorkbook().getSheetAt(0);
        updateCTTable(sheet, usedSeriesDatas);

        removeExtraSheetCell(sheet, data.getCategories().length, totalSeriesCount, usedSeriesDatas.size());

        for (XDDFChartData chartData : chartSeries) {
            plot(chart, chartData);
        }
        chart.setTitleText(data.getChartTitle());
        chart.setTitleOverlay(false);
    }