RFR: 8170794: [macosx] Error when using setDesktopManager on a JDesktopPane on MacOS X with Look and Feel Aqua

[Prasanta Sadhukhan]

When we call JDesktopPane.setDesktopManager(new DefaultDesktopManager()) and try to iconify an internalframe, it results in NPE

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javax.swing.RepaintManager.getVolatileOffscreenBuffer(RepaintManager.java:1030)
at javax.swing.RepaintManager$PaintManager.paint(RepaintManager.java:1489)
at javax.swing.RepaintManager.paint(RepaintManager.java:1272)
at javax.swing.JComponent.paint(JComponent.java:1042)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)
at com.apple.laf.AquaInternalFrameDockIconUI$ScaledImageLabel.updateIcon(AquaInternalFrameDockIconUI.java:193)
at com.apple.laf.AquaInternalFrameDockIconUI$ScaledImageLabel.paint(AquaInternalFrameDockIconUI.java:204)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)
at javax.swing.JComponent.paintChildren(JComponent.java:889)
at javax.swing.JComponent.paint(JComponent.java:1065)
at javax.swing.JLayeredPane.paint(JLayeredPane.java:586) 


it seems the getVolatileOffscreenBuffer() is called with JRootPane component for AquaLookAndFeel because of which 
SwingUtilities.getWindowAncestor(JRootPane) returns null causing NPE.

Fixed by adding a null check. The JInternalFrame can be successfully iconized now after setting DesktopManager.

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Commit messages:
 - Fix
 - Fix
 - 8170794: [macosx] Error when using setDesktopManager on a JDesktopPane on MacOS X with Look and Feel Aqua

Changes: https://git.openjdk.org/jdk/pull/9408/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9408&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8170794
  Stats: 106 lines in 2 files changed: 105 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9408.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9408/head:pull/9408

PR: https://git.openjdk.org/jdk/pull/9408