Warn on bad value passed to registerChild in WindowScroller

[TrySound]

No description provided.

[codecov-io]

Codecov Report

Merging #949 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #949      +/-   ##
==========================================
+ Coverage   90.37%   90.39%   +0.01%     
==========================================
  Files          57       57              
  Lines        1632     1634       +2     
==========================================
+ Hits         1475     1477       +2     
  Misses        157      157

Impacted Files	Coverage Δ
source/WindowScroller/WindowScroller.js	98.46% <100%> (+0.04%)	⬆️

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