Running the attached program will cause a crash on exit:

$ java GlassCrash
Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at GlassCrash$SizingTestApp.lambda$getTestDialog$2(GlassCrash.java:134)
at javafx.base/com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:247)
at javafx.base/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:234)
at javafx.base/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at javafx.base/com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at javafx.base/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at javafx.base/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at javafx.base/com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at javafx.base/com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.base/javafx.event.Event.fireEvent(Event.java:198)
at javafx.graphics/javafx.stage.Window.fireEvent(Window.java:1368)
at javafx.graphics/javafx.stage.Window$12.invalidated(Window.java:1127)
at javafx.base/javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:110)
at javafx.base/javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:145)
at javafx.graphics/javafx.stage.Window.setShowing(Window.java:1174)
at javafx.graphics/javafx.stage.Window.hide(Window.java:1199)
at javafx.graphics/com.sun.javafx.stage.WindowPeerListener.closed(WindowPeerListener.java:105)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassWindowEventHandler.run(GlassWindowEventHandler.java:144)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassWindowEventHandler.run(GlassWindowEventHandler.java:40)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassWindowEventHandler.lambda$handleWindowEvent$4(GlassWindowEventHandler.java:176)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithoutRenderLock(QuantumToolkit.java:412)
at javafx.graphics/com.sun.javafx.tk.quantum.GlassWindowEventHandler.handleWindowEvent(GlassWindowEventHandler.java:174)
at javafx.graphics/com.sun.glass.ui.Window.handleWindowEvent(Window.java:1351)
at javafx.graphics/com.sun.glass.ui.Window.notifyDestroy(Window.java:1260)
at javafx.graphics/com.sun.glass.ui.gtk.GtkWindow._close(Native Method)
at javafx.graphics/com.sun.glass.ui.Window.close(Window.java:352)
at javafx.graphics/com.sun.javafx.tk.quantum.WindowStage.lambda$close$4(WindowStage.java:822)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.runWithRenderLock(QuantumToolkit.java:430)
at javafx.graphics/com.sun.javafx.tk.quantum.WindowStage.close(WindowStage.java:815)
at javafx.graphics/javafx.stage.Window$12.invalidated(Window.java:1141)
at javafx.base/javafx.beans.property.BooleanPropertyBase.markInvalid(BooleanPropertyBase.java:110)
at javafx.base/javafx.beans.property.BooleanPropertyBase.set(BooleanPropertyBase.java:145)
at javafx.graphics/javafx.stage.Window.setShowing(Window.java:1174)
at javafx.graphics/javafx.stage.Window.hide(Window.java:1199)
at GlassCrash.lambda$exit$3(GlassCrash.java:101)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:277)
at java.base/java.lang.Thread.run(Thread.java:830)
#
# A fatal error has been detected by the Java Runtime Environment:
#
# SIGSEGV (0xb) at pc=0x00007fe41ac60d33, pid=30962, tid=30986
#
# JRE version: OpenJDK Runtime Environment (13.0.1+9) (build 13.0.1+9)
# Java VM: OpenJDK 64-Bit Server VM (13.0.1+9, mixed mode, sharing, tiered, compressed oops, g1 gc, linux-amd64)
# Problematic frame:
# C [libglassgtk3.so+0x1fd33] Java_com_sun_glass_ui_gtk_GtkWindow_setVisibleImpl+0x13
#
# Core dump will be written. Default location: Core dumps may be processed with "/usr/share/apport/apport %p %s %c %d %P" (or dumping to /localhome/kcr/javafx/tmp/core.30962)
#
# An error report file with more information is saved as:
# /localhome/kcr/javafx/tmp/hs_err_pid30962.log
#
# If you would like to submit a bug report, please visit:
# http://bugreport.java.com/bugreport/crash.jsp
# The crash happened outside the Java Virtual Machine in native code.
# See problematic frame for where to report the bug.
#
Aborted (core dumped)

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This is not a new issue, since it happens at least as far back as 8u60. It was uncovered while reviewing the unit test for JDK-8232811.