RFR: 8247163: Fixing exception when clicking on JFXPanel when no Scene is set

[Florian Kirmaier]

Fixing exception when clicking on JFXPanel when no Scene is set.

quick test: `./gradlew -PFULL_TEST=true -PUSE_ROBOT=true :systemTests: test --tests *javafx.embed.swing.JFXPan* --info`

It's a regression from my previous PR: https://github.com/openjdk/jfx/pull/25

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Commit messages:
 - 8247163:

Changes: https://git.openjdk.java.net/jfx/pull/248/files
 Webrev: https://webrevs.openjdk.java.net/jfx/248/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247163
  Stats: 22 lines in 2 files changed: 20 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jfx/pull/248.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/248/head:pull/248

PR: https://git.openjdk.java.net/jfx/pull/248