[jfx-tests] RFR: 8316097: Some Scenegraph/richtext tests fail due to IllegalStateException

[Ajit Ghaisas]

test/scenegraph/richtext/RichTextLabeledsTest.java
test/scenegraph/richtext/RichTextMixedTest.java
test/scenegraph/richtext/RichTextRectangleTest.java
test/scenegraph/richtext/RichTextTextTest.java

These tests were failing with -  java.lang.IllegalStateException: This operation is permitted on the event thread only; currentThread = Time-limited test. They are fixed with a change to single file `RichTextPropertiesApp.java`

Another test - test/scenegraph/richtext/RichTextSpecSymbTest.java - was failing due to NPE. Added a null check to fix it.

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Commit messages:
 - fix richtext tests

Changes: https://git.openjdk.org/jfx-tests/pull/9/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx-tests&pr=9&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316097
  Stats: 15 lines in 2 files changed: 11 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jfx-tests/pull/9.diff
  Fetch: git fetch https://git.openjdk.org/jfx-tests.git pull/9/head:pull/9

PR: https://git.openjdk.org/jfx-tests/pull/9