dChart addon, class not found error

Hello,

I got an exception when i use the dchart addon, class NumberUtil not found (picture joined).
Can you tell me how to resolve it, please?

DataSeries dataSeries = new DataSeries()
            .newSeries()
            .add("none", 23)
            .newSeries()
            .add("error", 10)
            .newSeries()
            .add("click", 5)
            .newSeries()
            .add("impression", 25);
        
        SeriesDefaults seriesDefaults = new SeriesDefaults()
            .setRenderer(SeriesRenderers.PIE)
            .setRendererOptions(
                new PieRenderer()
                    .setShowDataLabels(true));

        Legend legend = new Legend()
            .setShow(true)
            .setPlacement(LegendPlacements.OUTSIDE_GRID)
            .setLocation(LegendLocations.WEST)
            .setRenderer(LegendRenderers.ENHANCED)
            .setRendererOptions(
                new EnhancedLegendRenderer()
                    .setSeriesToggle(SeriesToggles.SLOW)
                    .setSeriesToggleReplot(true));

        Options options = new Options()
            .setSeriesDefaults(seriesDefaults)
            .setLegend(legend);

        DCharts chart = new DCharts()
            .setDataSeries(dataSeries)
            .setOptions(options)
            .setEnableDownload(true)
            .setChartImageFormat(ChartImageFormat.GIF)
            .show();
        chart.setWidth("300px");
        chart.setHeight("300px");
        addComponent(chart);
        setComponentAlignment(chart, Alignment.MIDDLE_CENTER);