RFR: 8322239: [macos] a11y : java.lang.NullPointerException is thrown when focus is moved on the JTabbedPane

[Alexander Zuev]

Add null check for the Aqua LnF situation when tab is hidden die to the tabs overflow.

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Commit messages:
 - 8322239: [macos] a11y : java.lang.NullPointerException is thrown when focus is moved on the JTabbedPane

Changes: https://git.openjdk.org/jdk/pull/17736/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17736&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322239
  Stats: 6 lines in 1 file changed: 3 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17736.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17736/head:pull/17736

PR: https://git.openjdk.org/jdk/pull/17736