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

[eduardsdv]

On Tue, 11 Jan 2022 14:42:19 GMT, eduardsdv <duke at openjdk.java.net> wrote:

> 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.

I added changes suggested in the review and added the file header to the test class.
Please review.

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