RFR: 8374515: javafx.scene.PropertyHelper causes uncessary silent NullPointerException

[Christopher Schnick]

This fixes a small issue where an NPE was always thrown on startup.

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Commit messages:
 - Remove ProperyHelper class

Changes: https://git.openjdk.org/jfx/pull/2023/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2023&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374515
  Stats: 41 lines in 2 files changed: 0 ins; 40 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/2023.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2023/head:pull/2023

PR: https://git.openjdk.org/jfx/pull/2023