RFR: 8373098: RichTextArea: NPE in InputMethodRequests

[Andy Goryachev]

Fixes an NPE discovered while working on https://github.com/openjdk/jfx/pull/1975

To reproduce, run RichTextAreaDemoApp with a "2,000,000,000 Lines" model and try scrolling with the mouse wheel rapidly up and down.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jfx/pull/1997/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1997&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373098
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jfx/pull/1997.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1997/head:pull/1997

PR: https://git.openjdk.org/jfx/pull/1997