RFR: 8244234: MenuButton: NPE on removing from scene with open popup [v2]

[eduardsdv]

> The NPE occurs when the skinnable is removed from the scene while the popup is showing.
> The MenuButtonSkinBase, when popup becomes hidden, tries to remove Mnemonics from the scene and runs into NPE.
> To avoid NPE a null-check is added to the 'showing' listener.
> 
> Since the mnemonics cannot be removed from the scene in standard way, when popup becomes hidden.
> They are now also removed from the scene, when the skinnable is removed from it.

eduardsdv has updated the pull request incrementally with one additional commit since the last revision:

  8244234: Add changes suggested in review and add file header to the test

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/713/files
  - new: https://git.openjdk.java.net/jfx/pull/713/files/0c598695..6749816f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=713&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=713&range=00-01

  Stats: 36 lines in 2 files changed: 25 ins; 0 del; 11 mod
  Patch: https://git.openjdk.java.net/jfx/pull/713.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/713/head:pull/713

PR: https://git.openjdk.java.net/jfx/pull/713