Apparent race between _closePopoverBlurHandler and popoverClosed

[shaoster]

I'm having trouble reproducing this myself with the latest version of this library and chrome 70.0.3538.110, but we have seen stack traces with failures on (null).querySelectorAll.

TypeError: Cannot read property 'querySelectorAll' of null
1
File ---.js line ### col ### in i.value
2
File ---.js line ### col ### in HTMLElement.e._closePopoverBlurHandler

[twokul]

the root cause of the issue if that willDestroyElement is never called causing blur event handler _closePopoverBlurHandler after this._popperElement is null-fied.

I'm not 100% sure why willDestroyElement is not called (confirmed it via running ember s and playing around with demo pages); the hacky fix would be to bail if this._popperElement === null

[shaoster]

Leaving this issue opened while we try to actually understand the root cause.