[jfx25] RFR: 8363813: Missing null check in GlassScreen

[Ambarish Rapte]

Clean backport of adding simple null checks in glass screen fix to the `jfx25` stabilization branch.

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Commit messages:
 - Backport dbd43a371a2460c3cf8aed108d349b301bd2a1b2

Changes: https://git.openjdk.org/jfx/pull/1854/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1854&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8363813
  Stats: 31 lines in 4 files changed: 10 ins; 4 del; 17 mod
  Patch: https://git.openjdk.org/jfx/pull/1854.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1854/head:pull/1854

PR: https://git.openjdk.org/jfx/pull/1854