RFR: 8255248: NullPointerException in JFXPanel due to race condition in HostContainer [v9]

[Prasanta Sadhukhan]

> NPE is seen while accessing transient "scenePeer" variable between reads..
> Fix is made to store it in a temp variable rather than reading it twice since the value can change between successive reads in many places it is accessed.
> Also some debug logs added to be enabled via `jfxpanel.debug` property

Prasanta Sadhukhan has updated the pull request incrementally with one additional commit since the last revision:

  Review comment + OutputRedirect

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1968/files
  - new: https://git.openjdk.org/jfx/pull/1968/files/f08b7417..f66fff5c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1968&range=08
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1968&range=07-08

  Stats: 21 lines in 2 files changed: 4 ins; 8 del; 9 mod
  Patch: https://git.openjdk.org/jfx/pull/1968.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1968/head:pull/1968

PR: https://git.openjdk.org/jfx/pull/1968