I'd expect there is a bug report on this already but my super-simple search
did not find it.
Run SwingSet2 on macos using JDK11 (or may be earlier) or even current JDK 16 with -Xcheck:jni and you'll get
WARNING in native method: JNI call made without checking exceptions when required to from CallStaticObjectMethod
        at sun.lwawt.macosx.LWCToolkit.initIDs(java.desktop@11.0.1/Native Method)
        at sun.lwawt.macosx.LWCToolkit.<clinit>(java.desktop@11.0.1/LWCToolkit.java:103)
        at java.lang.Class.forName0(java.base@11.0.1/Native Method)
        at java.lang.Class.forName(java.base@11.0.1/Class.java:315)
        at java.awt.Toolkit$2.run(java.desktop@11.0.1/Toolkit.java:588)
        at java.awt.Toolkit$2.run(java.desktop@11.0.1/Toolkit.java:583)
        at java.security.AccessController.doPrivileged(java.base@11.0.1/Native Method)
        at java.awt.Toolkit.getDefaultToolkit(java.desktop@11.0.1/Toolkit.java:582)
        - locked <0x000000070fea3048> (a java.lang.Class for java.awt.Toolkit)
        at java.awt.Toolkit.getEventQueue(java.desktop@11.0.1/Toolkit.java:1494)
        at java.awt.EventQueue.invokeLater(java.desktop@11.0.1/EventQueue.java:1312)
        at javax.swing.SwingUtilities.invokeLater(java.desktop@11.0.1/SwingUtilities.java:1421)
        at SwingSet2.main(SwingSet2.java:198)
Warning: Calling other JNI functions in the scope of Get/ReleasePrimitiveArrayCritical or Get/ReleaseStringCritical
Warning: Calling other JNI functions in the scope of Get/ReleasePrimitiveArrayCritical or Get/ReleaseStringCritical
<Last warning repeated thousands of times>