RFR: 8255248: NullPointerException in JFXPanel due to race condition in HostContainer

[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.

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Commit messages:
 - 8255248: NullPointerException in JFXPanel due to race condition in HostContainer

Changes: https://git.openjdk.org/jfx/pull/1178/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1178&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8255248
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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