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. ------------- 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
