fix(TransitionablePortal): fix onClose call

[mterezac]

Fixes #4056.

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Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.75%. Comparing base (bb2db59) to head (1fe1aac).
Report is 68 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4334   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files         180      180           
  Lines        3241     3243    +2     
=======================================
+ Hits         3233     3235    +2     
  Misses          8        8           

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