The following method in XYChart.java:

http://hg.openjdk.java.net/openjfx/9-dev/rt/file/9c2e8ff4fa26/modules/controls/src/main/java/javafx/scene/chart/XYChart.java#l910

Copied here for convenience:

/**
 * This should be called from seriesRemoved() when you are finished with any animation for deleting the series from
 * the chart. It will remove the series from showing up in the Iterator returned by getDisplayedSeriesIterator().
 *
 * @param series The series to remove
 */
protected final void removeSeriesFromDisplay(Series<X, Y> series) {
    if (series != null) series.setToRemove = false;
    series.setChart(null);
    displayedSeries.remove(series);
}

Seems like it could throw a NullPointerException if the given series argument is null. Perhaps it should actually be:

/**
 * This should be called from seriesRemoved() when you are finished with any animation for deleting the series from
 * the chart. It will remove the series from showing up in the Iterator returned by getDisplayedSeriesIterator().
 *
 * @param series The series to remove
 */
protected final void removeSeriesFromDisplay(Series<X, Y> series) {
    if (series != null) {
        series.setToRemove = false;
        series.setChart(null);
    }
    displayedSeries.remove(series);
}