RFR: 8372757: MacOS, Accessibility: Crash in [MenuAccessibility accessibilityChildren] after JDK-8341311

[Alexander Zuev]

Perform null check before requesting information for the popup component and return nil if component is no longer exists.

-------------

Commit messages:
 - 8372757: MacOS, Accessibility: Crash in [MenuAccessibility accessibilityChildren] after JDK-8341311

Changes: https://git.openjdk.org/jdk/pull/28614/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28614&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372757
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28614.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28614/head:pull/28614

PR: https://git.openjdk.org/jdk/pull/28614