FULL PRODUCT VERSION :
java version "1.6.0-ea"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-ea-b56)
Java HotSpot(TM) Client VM (build 1.6.0-ea-b56, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows XP [Version 5.1.2600]

A DESCRIPTION OF THE PROBLEM :
This is new function in MetalInternalFrameUI:
    protected MouseInputAdapter createBorderListener(JInternalFrame w) {
        return new BorderListener1();
    }

One of the functions in BorderListener1 is
        Rectangle getIconBounds() {
            boolean leftToRight = MetalUtils.isLeftToRight(frame);
            int xOffset = leftToRight ? 5 : titlePane.getWidth() - 5;
            Rectangle rect = null;

            Icon icon = frame.getFrameIcon();
            if ( icon != null ) {
                if ( !leftToRight ) {
                    xOffset -= icon.getIconWidth();
                }
>>> int iconY = ((titlePane.getHeight() / 2) - (icon.getIconHeight() /2));
                rect = new Rectangle(xOffset, iconY,
                    icon.getIconWidth(), icon.getIconHeight());
            }
            return rect;
        }

There's a NullPointerException in the line marked with >>> under custom look-and-feel that extends MetalInternalFrameUI and overrides createNorthPane function.

The cause -
1. titlePane variable is defined as private in MetalInternalFrameUI and as such, can not be set without ugly reflection in inheritor.
2. In order to create custom title pane, the createNorthPane() function must be overriden.
3. This will cause the titlePane variable in MetalInternalFrameUI to remain unassigned.
4. On mouse-click on frame title pane (only under Mustang), the newly added listener will be invoked, throwing a NPE since titlePane is null.

Note that this effectively breaks existing applications, forcing LAF writers to override function (createBorderListener) that didn't exist prior to Mustang.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. Create any application with DesktopPane and InternalFrame. Use Substance LAF from https://substance.dev.java.net/source/browse/substance/drop/. Must use version 1.65 or less (1.66 fixes the problem overriding the createBorderListener).
2. Double-click on frame icon or title.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Double-click on icon closes the frame.
Double-click on title maximizes the frame.
ACTUAL -
NullPointerException

ERROR MESSAGES/STACK TRACES THAT OCCUR :
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javax.swing.plaf.metal.MetalInternalFrameUI$BorderListener1.getIconBounds(MetalInternalFrameUI.java:206)
at javax.swing.plaf.metal.MetalInternalFrameUI$BorderListener1.mouseClicked(MetalInternalFrameUI.java:216)
at java.awt.AWTEventMulticaster.mouseClicked(AWTEventMulticaster.java:212)
at java.awt.Component.processMouseEvent(Component.java:5958)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3285)
at java.awt.Component.processEvent(Component.java:5720)
at java.awt.Container.processEvent(Container.java:1960)
at java.awt.Component.dispatchEventImpl(Component.java:4365)
at java.awt.Container.dispatchEventImpl(Container.java:2018)
at java.awt.Component.dispatchEvent(Component.java:4195)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4222)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3895)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3816)
at java.awt.Container.dispatchEventImpl(Container.java:2004)
at java.awt.Window.dispatchEventImpl(Window.java:2210)
at java.awt.Component.dispatchEvent(Component.java:4195)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)


REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Existing LAFs must override createBorderListener() function (new in Mustang).