Update ExporterButton.js

Removed:

        this.addEvents(
            'start',
            'complete');

No longer required in ExtJS 5 as Ext.util.Observable#addEvents" is deprecated.  Custom events can now be fired without prior addEvents.

Returns functionality to normal in ExtJS 5 and prevents below error for occurring:
Uncaught Error: "Ext.util.Observable # addEvents" is deprecated.