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

[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.

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Commit messages:
 - 8376492: NullPointer in ContextMenu sub-menu when graphic and style classes are changed while the menu is open

Changes: https://git.openjdk.org/jfx/pull/2075/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2075&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376492
  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 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