ButtonOptions onclick should be a Function

[swanthe]

Instead of a DummyOption, ButtonOptions.onclick should be a Function. That way you can add an event function to i.e. the ExportingButtons.contextButton.

From the Highcharts API reference exporting.buttons.contextButton

onclick: functionSince 2.0.0
A click handler callback to use on the button directly instead of the popup menu.

Expected generated code
exporting: { buttons: { contextButton: { menuItems: null, onclick: function () { this.exportChart(); } } } }