The documentation misleading. There is no getWorkbook function it is getWorkbookAsync.
Are you sure about this? I am having the same issue. When I inspect the Viz object I see that it has getWorkbook, not getWorkbookAsync.
Uncaught TypeError: Cannot call method 'get__workbook' of null tableau_v8.js:201
Uncaught TypeError: Object [object Object] has no method 'getWorkbookAsync'
Did you solve this problem?