[jfx21u] RFR: 8315074: Possible null pointer access in native glass

[Jayathirth D V]

This is jfx21u backport of https://bugs.openjdk.org/browse/JDK-8315074.
It adds appropriate null checks in glass code.

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Commit messages:
 - Backport f7b21e5468f1aad18df17443590c0887b2cf0239

Changes: https://git.openjdk.org/jfx21u/pull/15/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx21u&pr=15&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315074
  Stats: 105 lines in 8 files changed: 70 ins; 7 del; 28 mod
  Patch: https://git.openjdk.org/jfx21u/pull/15.diff
  Fetch: git fetch https://git.openjdk.org/jfx21u.git pull/15/head:pull/15

PR: https://git.openjdk.org/jfx21u/pull/15