RFR: 8255248: NullPointerException in JFXPanel due to race condition in HostContainer [v4]

[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:

  Remove debug, store transient var in temp var in EDT methods

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Changes:
  - all: https://git.openjdk.org/jfx/pull/1968/files
  - new: https://git.openjdk.org/jfx/pull/1968/files/115001f6..7bf8a91e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1968&range=03
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1968&range=02-03

  Stats: 155 lines in 1 file changed: 5 ins; 129 del; 21 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