RFR: 8255248: NullPointerException in JFXPanel due to race condition in HostContainer [v2]

[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 one additional commit since the last revision:

  Add lock for read/write of scenePeer/stagePeer and added system test

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1178/files
  - new: https://git.openjdk.org/jfx/pull/1178/files/f07c77ae..73a3e6b0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1178&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1178&range=00-01

  Stats: 224 lines in 2 files changed: 181 ins; 1 del; 42 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