RFR: 8327177: Wrong GlobalRef deleted

[Johan Vos]

Explicitly remove old global ref value from menu->jCallback.
This avoids the possibility of that value (which can be reused for other refs) to be deleted twice.

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Commit messages:
 - Explicitly remove old global ref value from menu->jCallback.

Changes: https://git.openjdk.org/jfx/pull/1388/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1388&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327177
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1388.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1388/head:pull/1388

PR: https://git.openjdk.org/jfx/pull/1388