Fix(Portal) check for rootNode rendering to prevent race condition

[Brantron]

We rely heavily on Semantic-UI-React and have had our monitoring reporting the following issue

TypeError: Cannot read property 'firstElementChild' of null

We've investigated it a few times but recently stumbled upon this issue #2401

This commit has been able to successfully prevent this issue without introducing any new issues.

[codecov-io]

Codecov Report

Merging #2457 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2457      +/-   ##
==========================================
- Coverage   99.74%   99.74%   -0.01%     
==========================================
  Files         154      154              
  Lines        2699     2694       -5     
==========================================
- Hits         2692     2687       -5     
  Misses          7        7

Impacted Files	Coverage Δ
src/addons/Portal/Portal.js	100% <100%> (ø)	⬆️
src/behaviors/Visibility/Visibility.js	100% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update aed4f01...204b94a. Read the comment docs.

[levithomason]

Let's add a failing test that validates the fix. The fix location is also suspect. If this.rootNode were null or undefined at line 364, where the check is currently, then line 354 would throw a TypeError before ever reaching that point:

    this.rootNode.className = className || ''

[Brantron]

@levithomason, excellent point. I dug a little further and came across facebook/react#9808 which highlights

The problem is unstable_renderSubtreeIntoContainer is async and can be called after componentDidUpdate. When refs are used in componentDidUpdate, they can be undefined due to async nature of unstable_renderSubtreeIntoContainer.

Thanks for the guidance. I've refactored a bit and added a test.

[Brantron]

@levithomason ping

[levithomason]

Ah, nice find! I've done some manual testing and this all looks great. Thanks for the detective work.

[aloey]

Thank you so much for resolving this issue!
Any chance you will be updating the npm package soon?

[levithomason]

Released in semantic-ui-react@v0.78.0.

[levithomason]

@aloey I shoot for a release every weekend. Sometimes life prevents it.

[JsGvDev]

Thanks folks to fix this error. I've spent a few hours to find out about this bug, because I only could get this error from Portal but into 'node_modules/react/cjs/react.development.js', until I can isolate that error. Nothing into the app is broken, everything was working fine, but I got that error #2401 from console. I was using semantic-ui-react@v0.76.0, after updating semantic-ui-react@v0.78.0 that error is gone.