ADDITIONAL SYSTEM INFORMATION :
We've seen it in macOS 10.12 and 10.14 so far
Java 1.8.0_161

A DESCRIPTION OF THE PROBLEM :
It looks like com.sun.glass.ui.Application#staticScreen_getScreens can return an empty array under certain circumstances, which results in an IndexOutOfBoundsException in com.sun.glass.ui.Screen#getMainScreen

java.lang.ArrayIndexOutOfBoundsException: 0
at java.util.Arrays$ArrayList.get(Arrays.java:3841)
at java.util.Collections$UnmodifiableList.get(Collections.java:1309)
at com.sun.glass.ui.Screen.getMainScreen(Screen.java:61)
at com.sun.javafx.tk.quantum.QuantumToolkit.getPrimaryScreen(QuantumToolkit.java:695)
at javafx.stage.Screen.updateConfiguration(Screen.java:92)
at javafx.stage.Screen.lambda$static$23(Screen.java:79)
at com.sun.javafx.tk.quantum.QuantumToolkit.notifyScreenListener(QuantumToolkit.java:691)
at com.sun.javafx.tk.quantum.QuantumToolkit.access$000(QuantumToolkit.java:135)
at com.sun.javafx.tk.quantum.QuantumToolkit$4.handleSettingsChanged(QuantumToolkit.java:676)
at com.sun.glass.ui.Screen.notifySettingsChanged(Screen.java:300)

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
We haven't found a way to reproduce it

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No crash
ACTUAL -
Crash

FREQUENCY : occasionally