See https://netbeans.org/bugzilla/show_bug.cgi?id=254257
The following exception occurs while debugging:

java.lang.IndexOutOfBoundsException: Index: 4, Size: 4
at java.util.ArrayList.rangeCheck(ArrayList.java:653)
at java.util.ArrayList.get(ArrayList.java:429)
at com.sun.tools.jdi.EventRequestManagerImpl.request(EventRequestManagerImpl.java:942)
at com.sun.tools.jdi.EventSetImpl$EventImpl.<init>(EventSetImpl.java:88)
at com.sun.tools.jdi.EventSetImpl$ThreadedEventImpl.<init>(EventSetImpl.java:175)
at com.sun.tools.jdi.EventSetImpl$ClassPrepareEventImpl.<init>(EventSetImpl.java:367)
at com.sun.tools.jdi.EventSetImpl.createEvent(EventSetImpl.java:741)
at com.sun.tools.jdi.EventSetImpl.build(EventSetImpl.java:628)
at com.sun.tools.jdi.EventQueueImpl.removeUnfiltered(EventQueueImpl.java:211)
at com.sun.tools.jdi.EventQueueImpl.removeInternal(EventQueueImpl.java:124)
at com.sun.tools.jdi.InternalEventHandler.run(InternalEventHandler.java:54)
[catch] at java.lang.Thread.run(Thread.java:745)

This is a synchronization issue. Patch that is attached to issue #JDK-6744127 should fix this race-condition.
Please consider fixing this in JDK 9 and JDK 8 patch.