ADDITIONAL SYSTEM INFORMATION :
macOS Mojave Version 10.14
Java Runtime 11.0.1
JavaFX 11

A DESCRIPTION OF THE PROBLEM :
Its difficult to track down exactly when this happens, but seems to consistently happen when switching focus between applications (Alt-Tab).  A popup appears (java has quit unexpectedly).  In the log, I see the following (seems to be triggering an assertion):

2018-11-05 09:03:42.204 java[5069:452703] unrecognized type is 4294967295
2018-11-05 09:03:42.204 java[5069:452703] *** Assertion failure in -[NSEvent _initWithCGEvent:eventRef:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/AppKit/AppKit-1671/AppKit.subproj/NSEvent.m:1969
2018-11-05 09:03:42.206 java[5069:452703] Apple AWT Internal Exception: Invalid parameter not satisfying: _type > 0 && _type <= kCGSLastEventType
2018-11-05 09:03:42.206 java[5069:452703] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid parameter not satisfying: _type > 0 && _type <= kCGSLastEventType'
*** First throw call stack:
(
	0   CoreFoundation                      0x00007fff2ed6243d __exceptionPreprocess + 256
	1   libobjc.A.dylib                     0x00007fff5ac6f720 objc_exception_throw + 48
	2   CoreFoundation                      0x00007fff2ed7d08e +[NSException raise:format:arguments:] + 98
	3   Foundation                          0x00007fff3111955d -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 194
	4   AppKit                              0x00007fff2c208acd -[NSEvent _initWithCGEvent:eventRef:] + 3272
	5   AppKit                              0x00007fff2c55c188 +[NSEvent eventWithCGEvent:] + 120
	6   libglass.dylib                      0x0000000110203f3b listenTouchEvents + 59
	7   SkyLight                            0x00007fff54d2d782 _ZL19processEventTapDataPvjjjPhj + 148
	8   SkyLight                            0x00007fff54c2cbb2 _XPostEventTapData + 278
	9   SkyLight                            0x00007fff54d2d690 _ZL22eventTapMessageHandlerP12__CFMachPortPvlS1_ + 132
	10  CoreFoundation                      0x00007fff2ecc0a5b __CFMachPortPerform + 246
	11  CoreFoundation                      0x00007fff2ecc0959 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE1_PERFORM_FUNCTION__ + 41
	12  CoreFoundation                      0x00007fff2ecc08b7 __CFRunLoopDoSource1 + 527
	13  CoreFoundation                      0x00007fff2eca8945 __CFRunLoopRun + 2574
	14  CoreFoundation                      0x00007fff2eca7ce4 CFRunLoopRunSpecific + 463
	15  Foundation                          0x00007fff310085da -[NSRunLoop(NSRunLoop) runMode:beforeDate:] + 280
	16  libglass.dylib                      0x00000001101f89f5 +[GlassApplication enterNestedEventLoopWithEnv:] + 165
	17  libglass.dylib                      0x00000001101f948a Java_com_sun_glass_ui_mac_MacApplication__1enterNestedEventLoopImpl + 74
	18  ???                                 0x0000000110444297 0x0 + 4567876247
)
libc++abi.dylib: terminating with uncaught exception of type NSException


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Start an application based on JavaFX
Switch context using Alt+Tab
Sometimes this causes a crash (quit unexpectedly)


CUSTOMER SUBMITTED WORKAROUND :
There is no workaround

FREQUENCY : often