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

[Ajit Ghaisas]

On Mon, 17 Jan 2022 15:32:13 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.
>
> 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

Marked as reviewed by aghaisas (Reviewer).

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