React 16.9+ failures: `initialFocus` refers to no known node

[wldcordeiro]

I've run into issues when updating to latest React where that error is thrown, primarily in test environments. It appears like we're checking for the initialFocus before it has rendered though when I run debug() in react-testing-library.

Working on getting a minimal reproduction case.

[toolness]

Ah I ran into this as well! I think it's actually reported in one of this library's dependencies at focus-trap/focus-trap-react#24. About a year ago, I documented my findings in a comment at focus-trap/focus-trap-react#24 (comment).