Vaadin ApexChartWrapper get dataURI() always null

Question

ApexchartWrapper from Vaadin directory works like a charm but I need to put the generated chart into a pdf.

But I did not get it running so far.

if I do

chart.dataURI().then(t -> {
    System.out.println("return" + t.asString());
});

This is executed after the chart is already shown on the UI.

The question is how to get the dataURI of the chart.