Issue with JScrollPane's updateUI call

[mgarin]

@Sciss mentioned on Gitter:

I'm running into this:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at javax.swing.ScrollPaneLayout.addSingletonComponent(ScrollPaneLayout.java:202)
    at javax.swing.ScrollPaneLayout.addLayoutComponent(ScrollPaneLayout.java:254)
    at java.awt.Container.addImpl(Container.java:1132)
    at java.awt.Container.add(Container.java:975)
    at javax.swing.JScrollPane.setCorner(JScrollPane.java:1267)
    at com.alee.laf.scroll.WebScrollPaneUI.updateCorners(WebScrollPaneUI.java:575)
    at com.alee.laf.scroll.WebScrollPaneUI.installUI(WebScrollPaneUI.java:94)
    at javax.swing.JComponent.setUI(JComponent.java:666)
    at javax.swing.JScrollPane.setUI(JScrollPane.java:380)
    at javax.swing.JScrollPane.updateUI(JScrollPane.java:393)
    at javax.swing.SwingUtilities.updateComponentTreeUI0(SwingUtilities.java:1238)
    at javax.swing.SwingUtilities.updateComponentTreeUI0(SwingUtilities.java:1253)
    at javax.swing.SwingUtilities.updateComponentTreeUI(SwingUtilities.java:1229)
    at bibliothek.gui.dock.themes.BasicTheme.updateUI(BasicTheme.java:222)
    at bibliothek.gui.dock.themes.BasicTheme.install(BasicTheme.java:187)
    at bibliothek.gui.dock.themes.BasicTheme.install(BasicTheme.java:170)
    at bibliothek.gui.dock.themes.ThemeManager.setTheme(ThemeManager.java:278)
    at bibliothek.gui.DockController.setTheme(DockController.java:788)
    at bibliothek.gui.DockController.initiate(DockController.java:391)
    at bibliothek.gui.DockController.<init>(DockController.java:275)
    at bibliothek.gui.DockController.<init>(DockController.java:260)
    at bibliothek.gui.dock.common.intern.CDockController.<init>(CDockController.java:47)
    at bibliothek.gui.dock.common.intern.EfficientControlFactory.createController(EfficientControlFactory.java:49)
    at bibliothek.gui.dock.common.CControl.init(CControl.java:497)
    at bibliothek.gui.dock.common.CControl.<init>(CControl.java:475)
    at bibliothek.gui.dock.common.CControl.<init>(CControl.java:459)
    at bibliothek.gui.dock.common.CControl.<init>(CControl.java:404)
    at bibliothek.gui.dock.common.CControl.<init>(CControl.java:381)
    at de.sciss.synth.swing.Main$.dockCtrl$lzycompute(Main.scala:195)

Seems that the caching of scroll pane corners is buggy, because their parents might be null (after previous removal) and this is not handled by ScrollPaneLayout

it's that the docking framework (https://github.com/Benoker/DockingFrames) calls updateComponentTreeUI because it has it's own skinning system. So this after WebLaF has been initialized.

a workaround might be to install a sub-class of ScrollPaneLayout that handles the case where oldC.getParent == null

[Sciss]

Here my temporary work-around: https://github.com/Sciss/Submin/blob/b404264e83ba206daa3d1ac011b939797cde23f1/src/main/java/de/sciss/submin/SubminScrollPaneUI.java#L12

[mgarin]

I've fixed this NPE along with a few internal issues that weren't really affecting anything visually.