RFR: 8376492: NullPointer in ContextMenu sub-menu when graphic and style classes are changed while the menu is open [v2]

[Dmitry Markov]

> A NullPointerException occurs when a ContextMenu submenu is opened for the second time and the graphic and style classes are modified while the menu is open.
> 
> Fix: Added a null check.

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

  Remove previous ItemStyleClassListener

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/2075/files
  - new: https://git.openjdk.org/jfx/pull/2075/files/3a204d74..08789775

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=2075&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=2075&range=00-01

  Stats: 10 lines in 1 file changed: 6 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/2075.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2075/head:pull/2075

PR: https://git.openjdk.org/jfx/pull/2075