NPE on tooltips with BitmapEncoder.saveBitmap

[jc7447]

When upgrading from 3.6.6 to 3.8.8 , BitmapEncoder.saveBitmap throws NPE when tooltips are used.

Caused by: java.lang.NullPointerException: null
        at org.knowm.xchart.internal.chartpart.PlotContent_XY.doPaint(PlotContent_XY.java:304) ~[xchart-3.8.8.jar:na]
        at org.knowm.xchart.internal.chartpart.PlotContent_.paint(PlotContent_.java:61) ~[xchart-3.8.8.jar:na]
        at org.knowm.xchart.internal.chartpart.Plot_.paint(Plot_.java:37) ~[xchart-3.8.8.jar:na]
        at org.knowm.xchart.internal.chartpart.Plot_AxesChart.paint(Plot_AxesChart.java:34) ~[xchart-3.8.8.jar:na]
        at org.knowm.xchart.XYChart.paint(XYChart.java:417) ~[xchart-3.8.8.jar:na]
        at org.knowm.xchart.BitmapEncoder.getBufferedImage(BitmapEncoder.java:277) ~[xchart-3.8.8.jar:na]
        at org.knowm.xchart.BitmapEncoder.saveBitmap(BitmapEncoder.java:81) ~[xchart-3.8.8.jar:na]
        at org.knowm.xchart.BitmapEncoder.saveBitmap(BitmapEncoder.java:65) ~[xchart-3.8.8.jar:na]

public class LineChart01 {

    private static final String CHART_PREFIX = "chart-";

    private static final String PNG_SUFFIX = ".png";

    private static final String CHARTS = "charts-";

    public void getChart() throws IOException {

        // generates Log data
        List<Integer> xData = new ArrayList<Integer>();
        List<Double> yData = new ArrayList<Double>();
        for (int i = -3; i <= 3; i++) {
            xData.add(i);
            yData.add(Math.pow(10, i));
        }

        // Create Chart
        XYChart chart = new XYChartBuilder().width(800).height(600).title("Powers of Ten").xAxisTitle("Power").yAxisTitle("Value").build();

        // Customize Chart
        chart.getStyler().setChartTitleVisible(true);
        chart.getStyler().setLegendPosition(LegendPosition.InsideNW);
        chart.getStyler().setYAxisLogarithmic(true);
        chart.getStyler().setXAxisLabelRotation(45);

        chart.getStyler().setToolTipsEnabled(true);
        chart.getStyler().setToolTipsAlwaysVisible(true);
        chart.getStyler().setToolTipType(Styler.ToolTipType.yLabels);
        chart.getStyler().setToolTipBorderColor(Color.RED);

        // Series
        chart.addSeries("10^x", xData, yData);

        Path tempFolder = Files.createTempDirectory(CHARTS);

        Path chartFile = Files.createTempFile(tempFolder, CHART_PREFIX, PNG_SUFFIX);

        System.out.println(chartFile);

        BitmapEncoder.saveBitmap(chart, chartFile.toString(), BitmapEncoder.BitmapFormat.PNG);
    }
}

This was working fine in 3.6.6 :