RFR: 8376169: JPopupMenu.setInvoker(null) causes NPE

[Prasanta Sadhukhan]

Invoking `JPopupMenu.setInvoker(null)` causes NPE which is fixed in this PR

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Commit messages:
 - 8376169: JPopupMenu.setInvoker(null) causes NPE

Changes: https://git.openjdk.org/jdk/pull/29377/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29377&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376169
  Stats: 17 lines in 2 files changed: 5 ins; 2 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/29377.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29377/head:pull/29377

PR: https://git.openjdk.org/jdk/pull/29377