RFR: 8315074: Possible null pointer access in native glass

[Jayathirth D V]

At multiple places in native glass code we don't have appropriate NULL checks which might result in null pointer access.

Added appropriate checks and all test run is green.

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Commit messages:
 - 8315074: Possible null pointer access in native glass

Changes: https://git.openjdk.org/jfx/pull/1223/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1223&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/jfx/pull/1223.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1223/head:pull/1223

PR: https://git.openjdk.org/jfx/pull/1223