If you create a jpackage app-image with --java-options option to define a splash screen, and if you have a jdk/bin in your path when launching the app, you get the exception.

sample:
prerequsite:
"input" is a directory containing the files splash.png and SwingSet2_11.jar
--------------
export PATH=
.../jdk-16/bin/jpackage --app-image -name ss16 --main-jar --java-options '-splash:$APPDIR/splash.png' \
export PATH=.../jdk-16/bin
./ss16/ss16
---------------
running ss16/ss16 results in splash screen showing, then:
Exception in thread "AWT-EventQueue-0" java.lang.InternalError: java.lang.reflect
.InvocationTargetException
        at java.desktop/sun.font.FontManagerFactory$1.run(Unknown Source)
        at java.base/java.security.AccessController.doPrivileged(Unknown Source)
        at java.desktop/sun.font.FontManagerFactory.getInstance(Unknown Source)
        at java.desktop/sun.font.SunFontManager.getInstance(Unknown Source)
        at java.desktop/sun.font.FontDesignMetrics.getMetrics(Unknown Source)
        at java.desktop/sun.swing.SwingUtilities2.getFontMetrics(Unknown Source)
        at java.desktop/javax.swing.JComponent.getFontMetrics(Unknown Source)
        at java.desktop/javax.swing.plaf.basic.BasicGraphicsUtils.getPreferredBut
tonSize(Unknown Source)
        at java.desktop/javax.swing.plaf.basic.BasicButtonUI.getPreferredSize(Unk
nown Source)
        at java.desktop/javax.swing.JComponent.getPreferredSize(Unknown Source)
        at java.desktop/javax.swing.plaf.metal.MetalDesktopIconUI.installComponen
ts(Unknown Source)
        at java.desktop/javax.swing.plaf.basic.BasicDesktopIconUI.installUI(Unkno
wn Source)
        at java.desktop/javax.swing.JComponent.setUI(Unknown Source)
        at java.desktop/javax.swing.JInternalFrame$JDesktopIcon.setUI(Unknown Sou
rce)
        at java.desktop/javax.swing.JInternalFrame$JDesktopIcon.updateUI(Unknown
Source)
        at java.desktop/javax.swing.JInternalFrame$JDesktopIcon.<init>(Unknown So
urce)
        at java.desktop/javax.swing.JInternalFrame.<init>(Unknown Source)
        at java.desktop/javax.swing.JInternalFrame.<init>(Unknown Source)
        at InternalFrameDemo.createInternalFramePalette(InternalFrameDemo.java:17
4)
        at InternalFrameDemo.<init>(InternalFrameDemo.java:120)
        at SwingSet2.preloadFirstDemo(SwingSet2.java:607)
        at SwingSet2.<init>(SwingSet2.java:183)
        at SwingSet2.lambda$main$1(SwingSet2.java:204)
        at java.desktop/java.awt.event.InvocationEvent.dispatch(Unknown Source)
        at java.desktop/java.awt.EventQueue.dispatchEventImpl(Unknown Source)
        at java.desktop/java.awt.EventQueue$4.run(Unknown Source)
        at java.desktop/java.awt.EventQueue$4.run(Unknown Source)
        at java.base/java.security.AccessController.doPrivileged(Unknown Source)
        at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doInte
rsectionPrivilege(Unknown Source)
        at java.desktop/java.awt.EventQueue.dispatchEvent(Unknown Source)
        at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(Unkno
wn Source)
        at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(Unknown
Source)
        at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(Unkno
wn Source)
        at java.desktop/java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.desktop/java.awt.EventDispatchThread.pumpEvents(Unknown Source)
        at java.desktop/java.awt.EventDispatchThread.run(Unknown Source)
Caused by: java.lang.reflect.InvocationTargetException
        at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstan
ce0(Native Method)
        at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstan
ce(Unknown Source)
        at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newIn
stance(Unknown Source)
        at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Unknown
Source)
        at java.base/java.lang.reflect.Constructor.newInstance(Unknown Source)
        ... 36 more
Caused by: java.lang.InternalError: platform encoding not initialized
        at java.desktop/sun.awt.Win32FontManager.getFontPath(Native Method)
        at java.desktop/sun.font.SunFontManager.getPlatformFontPath(Unknown Sourc
e)
        at java.desktop/sun.font.SunFontManager.getPlatformFontDirs(Unknown Sourc
e)
        at java.desktop/sun.awt.Win32FontManager.getDefaultPlatformFont(Unknown S
ource)
        at java.desktop/sun.font.SunFontManager$2.run(Unknown Source)
        at java.desktop/sun.font.SunFontManager$2.run(Unknown Source)
        at java.base/java.security.AccessController.doPrivileged(Unknown Source)
        at java.desktop/sun.font.SunFontManager.<init>(Unknown Source)
        at java.desktop/sun.awt.Win32FontManager.<init>(Unknown Source)
        ... 41 more

--------------
without the "export PATH=.../jdk-16/bin" SwingSet2 runs fine, splash shows, then SwingSet2 shows.