[Rev 01] RFR: 8244110: NPE in MenuButtonSkinBase change listener

[Ajit Ghaisas]

> Issue :
> https://bugs.openjdk.java.net/browse/JDK-8244110
> 
> Root Cause :
> Fix of [JDK-8175358](https://bugs.openjdk.java.net/browse/JDK-8175358) added code to remove accelerators from a scene
> in Scene property listener of MenuButtonSkinBase. That fix uses getSkinnable() in listener. It turned out that it can
> be null as found out by failing system test.  Fix :
> As pointed out by @kleopatra on the JBS, getSkinnable() should never be null for a valid skin. Hence, the correct fix
> is to remove listener cleanly in dispose method. See JBS comments for more info. This also addresses
> [JDK-8244081](https://bugs.openjdk.java.net/browse/JDK-8244081).  Testing :
> 1) Failing system test - passes after the fix
> 2) 3 unit tests which used to log NPE message silently and pass have been made to fail without this fix - these tests
> passes after the fix
> Special Thanks to @kleopatra for most of the test digging and fix guidance.

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

  Make ChangeListener Final

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/205/files
  - new: https://git.openjdk.java.net/jfx/pull/205/files/66a19546..d0c498e0

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/205/webrev.01
 - incr: https://webrevs.openjdk.java.net/jfx/205/webrev.00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/205.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/205/head:pull/205

PR: https://git.openjdk.java.net/jfx/pull/205