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
------------- Commit messages: - 8255248: NullPointerException in JFXPanel due to race condition in HostContainer Changes: https://git.openjdk.org/jfx/pull/1968/files Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1968&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8255248 Stats: 208 lines in 3 files changed: 182 ins; 0 del; 26 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
