The getFormContext function is not implemented in onSave event

[mike9182]

When an onSave event is fired, the "getFormContext" method is not implemented in the returned event.

The code below throws an Error with the message "getFormContext not implemented.".

let formContext: Xrm.FormContext;
entity.addOnSave((saveContext: Xrm.Events.SaveEventContext) => {
    formContext = saveContext.getFormContext();
});

entity.save();