RFR: 8355415: RichTextArea: NPE in VFlow::scrollCaretToVisible [v2]

[Andy Goryachev]

> Fixed an NPE that should only happen in a headless test environment.
> 
> This is a follow-up for https://github.com/openjdk/jfx/pull/1677

Andy Goryachev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge remote-tracking branch 'origin/master' into 8355415.scroll.to.visible
 - on windows
 - Merge remote-tracking branch 'origin/master' into 8355415.scroll.to.visible
 - npe

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

Changes:
  - all: https://git.openjdk.org/jfx/pull/1793/files
  - new: https://git.openjdk.org/jfx/pull/1793/files/4dc2ad46..3109fb45

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jfx&pr=1793&range=01
 - incr: https://webrevs.openjdk.org/?repo=jfx&pr=1793&range=00-01

  Stats: 196 lines in 11 files changed: 169 ins; 7 del; 20 mod
  Patch: https://git.openjdk.org/jfx/pull/1793.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1793/head:pull/1793

PR: https://git.openjdk.org/jfx/pull/1793