[jfx17u] RFR: 8244234: MenuButton: NPE on removing from scene with open popup

[Johan Vos]

clean backport of 8244234: MenuButton: NPE on removing from scene with open popup
Reviewed-by: aghaisas

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Commit messages:
 - 8244234: MenuButton: NPE on removing from scene with open popup

Changes: https://git.openjdk.org/jfx17u/pull/69/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx17u&pr=69&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8244234
  Stats: 131 lines in 2 files changed: 126 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jfx17u/pull/69.diff
  Fetch: git fetch https://git.openjdk.org/jfx17u pull/69/head:pull/69

PR: https://git.openjdk.org/jfx17u/pull/69