A DESCRIPTION OF THE PROBLEM :
In "javafx.graphics/javafx.stage.Window$TKBoundsConfigurator.apply(...)" is missing the check that the peer is Null for the call "peer.setBounds(...);"
I run in situations where the peer is Null.

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at javafx.graphics/javafx.stage.Window$TKBoundsConfigurator.apply(Window.java:1550)
at javafx.graphics/javafx.stage.Window$TKBoundsConfigurator.pulse(Window.java:1559)
at javafx.graphics/com.sun.javafx.tk.Toolkit.lambda$runPulse$2(Toolkit.java:412)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at javafx.graphics/com.sun.javafx.tk.Toolkit.runPulse(Toolkit.java:411)
at javafx.graphics/com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:435)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:562)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:542)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.pulseFromQueue(QuantumToolkit.java:535)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.lambda$runToolkit$11(QuantumToolkit.java:341)
at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Use org.controlsfx.samples.HelloNotifications from https://github.com/controlsfx/controlsfx/blob/master/controlsfx-samples/src/main/java/org/controlsfx/samples/HelloNotifications.java and press very frequent on "Top-right notification" then the NPE will occur. It is independent of the os. We tested it on windows and mac.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The NPE should not occur in in javafx.graphics/javafx.stage.Window$TKBoundsConfigurator.apply(...).
ACTUAL -
The NPE occurred.

CUSTOMER SUBMITTED WORKAROUND :
No workaround known at the moment.

FREQUENCY : often