See the report: http://mail.openjdk.java.net/pipermail/swing-dev/2014-November/004018.html

There is the exception in the Window.setAlwaysOnTop() method:

java.util.ConcurrentModificationException
            at java.util.Vector$Itr.checkForComodification(Unknown Source)
            at java.util.Vector$Itr.next(Unknown Source)
            at java.awt.Window.setAlwaysOnTop(Unknown Source)
            at de.eurodata.commons.swing.SwingUtils$1.run(SwingUtils.java:344)
            at java.awt.event.InvocationEvent.dispatch(Unknown Source)
            at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
            at java.awt.EventQueue.access$400(Unknown Source)
            at java.awt.EventQueue$3.run(Unknown Source)
            at java.awt.EventQueue$3.run(Unknown Source)
            at java.security.AccessController.doPrivileged(Native Method)
            at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
            at java.awt.EventQueue.dispatchEvent(Unknown Source)
            at de.eurodata.commons.swing.event.AWTThrowableHandler.dispatchEvent(AWTThrowableHandler.java:28)
            at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
            at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
            at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
            at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
            at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
            at java.awt.EventDispatchThread.run(Unknown Source)