addSeriesAsDrilldown cause NPE if triggered before manual charts drilldown

Steps to Reproduce

Run attached sample.

Current Result

Click button > cause NPE

Click chart's drillable serie, click back to button to return to root

Click button > chart drilldown as expected

Expected Result

addSeriesAsDrilldown doesn't cause NPE

Debug Information

Charts internal org.zkoss.chart.Charts._drilldownLevels list is null until initialized in org.zkoss.chart.Charts.service(AuRequest, boolean)

in the org.zkoss.chart.ChartsEvents.ON_PLOT_DRILL_DOWN section.

If the chart has not triggered org.zkoss.chart.ChartsEvents.ON_PLOT_DRILL_DOWN at least one time, calling addSeriesAsDrilldown causes a NPE instead.

Workaround