RFR: 8363813: Missing null check in GlassScreen

[Ambarish Rapte]

The JNI method NewObjectArray may return null, so a null check should be added as a precaution in methods that creates an array of Screens.

-------------

Commit messages:
 - null check for NewObjectArray

Changes: https://git.openjdk.org/jfx/pull/1851/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1851&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/1851.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1851/head:pull/1851

PR: https://git.openjdk.org/jfx/pull/1851