ADDITIONAL SYSTEM INFORMATION :
Windows 10, JDK13.0.1, JavaFX 15-ea+2

A DESCRIPTION OF THE PROBLEM :
Hello!

I wrote a JavaFX-App pinging several devices. The status of the devices is monitored by circles (here "lamps"). - No response: circle gets red; response: circle gets green. This is provided by calling the call-method in a class extending Task. It updates the color of the circle-attribute by the setFill-method.

The program is able to run for hours, but somewhen the ui crashes, while pining goes on:

Exception in thread "JavaFX Application Thread" java.lang.NullPointerException
at javafx.scene.Scene$ScenePulseListener.synchronizeSceneNodes(Scene.java:2393)
at javafx.scene.Scene$ScenePulseListener.pulse(Scene.java:2540)
at com.sun.javafx.tk.Toolkit.lambda$runPulse$2(Toolkit.java:412)
at java.base/java.security.AccessController.doPrivileged(AccessController.java:391)
at com.sun.javafx.tk.Toolkit.runPulse(Toolkit.java:411)
at com.sun.javafx.tk.Toolkit.firePulse(Toolkit.java:438)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:563)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulse(QuantumToolkit.java:543)
at com.sun.javafx.tk.quantum.QuantumToolkit.pulseFromQueue(QuantumToolkit.java:536)
at com.sun.javafx.tk.quantum.QuantumToolkit.lambda$runToolkit$11(QuantumToolkit.java:342)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.lambda$runLoop$3(WinApplication.java:174)
at java.base/java.lang.Thread.run(Thread.java:830)

Snippet of the call()-method in the extending class:

while(App.isRunning())
{
   for(Device device : deviceList)
    {
InetAddress geek;

try {
geek = InetAddress.getByName(device.getIp());
if (geek.isReachable(3000)) {
device.getLamp().setFill(Paint.valueOf("66FF00"));
} else {
device.getLamp().setFill(Paint.valueOf("E30022"));
}
} catch (IOException e) {
e.printStackTrace();
}
     }
 }



FREQUENCY : always