fix: can't access property "style", getTextArea() is undefined

[afc163]

ant-design/ant-design#43615 (comment)

[codecov]

Codecov Report

Merging #50 (53e9a81) into master (38bf68e) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 53e9a81 differs from pull request most recent head 5001272. Consider uploading reports for the commit 5001272 to get more accurate results

@@           Coverage Diff           @@
##           master      #50   +/-   ##
=======================================
  Coverage   99.03%   99.03%           
=======================================
  Files           3        3           
  Lines         207      207           
  Branches       57       57           
=======================================
  Hits          205      205           
  Misses          2        2           

Files	Coverage Δ
src/TextArea.tsx	100.00% <100.00%> (ø)

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