RFR: 8264330: Scene MouseHandler is referencing removed nodes

[John Hendrikx]

Small fix to clear a reference to a removed node left by Scene$MouseHandler.

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Commit messages:
 - 8264330: Scene MouseHandler is referencing removed nodes

Changes: https://git.openjdk.java.net/jfx/pull/448/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=448&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264330
  Stats: 52 lines in 2 files changed: 51 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/448.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/448/head:pull/448

PR: https://git.openjdk.java.net/jfx/pull/448