Fix TableViewportSubscription race condition

[mofojed]

• Race condition was causing UI error with selectDistinct
• TableViewportSubscription could initialize after it had already been closed
• Check the status to see if we're already DONE before initializing
• Fixes Using the Web GUI's "Select Distinct" function results in an error. #3251

Race condition occurs when subscription.close() is called before the subscription has copied the original table. You can reproduce this in a JS API page in the developer console by doing a setInterval, e.g.:

setInterval(() => {
    var subscription = table.setViewport(0, 10)
    subscription.close()
    console.log('table size', table.size)
}, 50)

[devinrsmith]

I think @niloc132 should review this.