Allow for async popover refs

[Blackbaud-SteveBrush]

I updated the demo component to show the use case for this: if theskyPopover reference doesn't exist on the DOM, it throws an error. This pull request should allow for asynchronous references.

#1473

[codecov-io]

Codecov Report

Merging #1475 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1475   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files         383     383           
  Lines        7416    7440   +24     
  Branches      948     951    +3     
======================================
+ Hits         7416    7440   +24

Impacted Files	Coverage Δ
src/modules/popover/popover.directive.ts	100% <100%> (ø)	⬆️

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