RFR: 6529151: NullPointerException in swing.plaf.synth.SynthLookAndFeel$Handler

[Prasanta Sadhukhan]

NPE is thrown for closed/disposed JDialogs after uninstalling the UI which is because for SynthEditorPaneUI even though is disposed of, Nimbus propertyHandler tries to get the style of this disposed component resulting in NPE, 
which is fixed by a null check to prevent deferencing style object to get the color.

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Commit messages:
 - 6529151: NullPointerException in swing.plaf.synth.SynthLookAndFeel

Changes: https://git.openjdk.org/jdk/pull/10167/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10167&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6529151
  Stats: 129 lines in 2 files changed: 129 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10167.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10167/head:pull/10167

PR: https://git.openjdk.org/jdk/pull/10167