RFR: 8255248: NullPointerException in JFXPanel due to race condition in HostContainer [v5]

[Prasanta Sadhukhan]

> Due to transient datatype of scenePeer, it can become null which can result in NPE in scenarios where scene is continuously been reset and set, which warrants a null check, as is done in other places for the same variable.

Prasanta Sadhukhan has updated the pull request incrementally with two additional commits since the last revision:

 - Locking for all shared resource between Swing and FX thread
 - Locking for all shared resource between Swing and FX thread

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1178/files
  - new: https://git.openjdk.org/jfx/pull/1178/files/f078f0a2..595f40d6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1178&range=04
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1178&range=03-04

  Stats: 116 lines in 1 file changed: 71 ins; 0 del; 45 mod
  Patch: https://git.openjdk.org/jfx/pull/1178.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1178/head:pull/1178

PR: https://git.openjdk.org/jfx/pull/1178