RFR: 8237372: NullPointerException in TabPaneSkin.stopDrag

[Robert Lichtenberger]

Test simulates a single mouse-released event.
Fix simply guards against the null case.

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Commits:
 - 923a63b2: 8237372: NullPointerException in TabPaneSkin.stopDrag

Changes: https://git.openjdk.java.net/jfx/pull/89/files
 Webrev: https://webrevs.openjdk.java.net/jfx/89/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8237372
  Stats: 29 lines in 2 files changed: 21 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jfx/pull/89.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/89/head:pull/89

PR: https://git.openjdk.java.net/jfx/pull/89