Fixed crash on categories collapse, document sidebar close, resize from desktop to mobile

[jorgefilipecosta]

In HierarchicalTermSelector we were not assigning the promise to this.fetchRequest as componentWillUnmount expected.
So each time componentWillUnmount was called the app crashed.

Fixes: #5637

How Has This Been Tested?

Verify it is possible to open and collapse categories panel without any problem.
Verify we can close sidebar with/without categories collapsed, and verify closing sidebar when resizing to mobile also works as expected.
Try to add a category as it generates a different request and see it is still possible to collapse the panel.

[aduth]

Expected to have been caused by #5450 (cc @mkaz). Which means, based on the intention of allowing this to more easily allow wp.apiRequest with promise, means that we'll have the same issue for that use-case, since (as may be known), promises are not cancelable, and Promise#abort does not exist.

Really, it speaks to a bigger problem of this approach to making network requests within a component. One which I'd hope will become resolved with changes in direction like #5219

[aduth]

👍

Seems we could also consider unsetting this.fetchRequest when the request is completed, so we're not needlessly causing abort (not that I expect it has much overhead).

[jorgefilipecosta]

Thank you for the review @aduth, I applied your suggestion of unsetting the variable when the requests are finished.

[mkaz]

@aduth - yeh, I had to add in checks for .abort() existing to avoid the errors.