chart.ref refers to an empty chart after calling chart.ref.exportChartLocal

[julamb]

[Stackblitz reproduction]

As seen in the example (with the console open): after clicking the export button once, chart.ref gets replaced by an empty chart object (even though the chart.ref$ observable does not emit a new value).

Consequently, clicking the export button again will yield the following error : TypeError: Cannot read property 'exporting' of undefined, and all chart interactions via chart.ref will be broken.

This looks like it could be related to #316.