When I want to delete a data series from a LineChart I get java.lang.NullPointerException at javafx.scene.chart.LineChart.seriesRemoved(Unknown Source) but it does not happen if the lineChart.serCreateSymbols() is set to true. I happens only for no symbols visible. If I enable the symbols just befeore deleting data series, there is no exception.
To reproduce the bug it is enough to add this line:
lc.setCreateSymbols(false);
in createChart() method in AdvancedLineChartSample.java in sample JavaFX program - Ensemble.

If you have any further questions I would be glad to help.