RFR: 8349004: DatePicker: NPE in show() when initialized in a background thread

[Andy Goryachev]

## Root Cause

Focus is being requested in show(), even a background thread.

## Solution

Do not request focus if in a background thread.

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Commit messages:
 - date picker fix

Changes: https://git.openjdk.org/jfx/pull/1708/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1708&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349004
  Stats: 32 lines in 2 files changed: 13 ins; 14 del; 5 mod
  Patch: https://git.openjdk.org/jfx/pull/1708.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1708/head:pull/1708

PR: https://git.openjdk.org/jfx/pull/1708