Create a TextField, call .setText(null); (or more realistically, bind it to a StringProperty that has not been initialized and therefore contains the value null).
Then focus the text field and press the left or right arrow keys, resulting in this exception:

java.lang.IllegalArgumentException: paragraph is null
at java.text.Bidi.<init>(Bidi.java:97)
at com.sun.javafx.scene.control.behavior.TextInputControlBehavior.getBidi(TextInputControlBehavior.java:271)
at com.sun.javafx.scene.control.behavior.TextInputControlBehavior.isMixed(TextInputControlBehavior.java:280)
at com.sun.javafx.scene.control.behavior.TextInputControlBehavior.nextCharacterVisually(TextInputControlBehavior.java:297)
at com.sun.javafx.scene.control.behavior.TextInputControlBehavior.callAction(TextInputControlBehavior.java:185)
at com.sun.javafx.scene.control.behavior.BehaviorBase.callActionForEvent(BehaviorBase.java:222)
at com.sun.javafx.scene.control.behavior.TextInputControlBehavior.callActionForEvent(TextInputControlBehavior.java:137)
at com.sun.javafx.scene.control.behavior.BehaviorBase$1.handle(BehaviorBase.java:136)
at com.sun.javafx.scene.control.behavior.BehaviorBase$1.handle(BehaviorBase.java:133)
at com.sun.javafx.event.CompositeEventHandler$NormalEventHandlerRecord.handleBubblingEvent(CompositeEventHandler.java:218)
at com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(CompositeEventHandler.java:80)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:238)
at com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(EventHandlerManager.java:191)
at com.sun.javafx.event.CompositeEventDispatcher.dispatchBubblingEvent(CompositeEventDispatcher.java:59)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:58)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(BasicEventDispatcher.java:56)
at com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(EventDispatchChainImpl.java:114)
at com.sun.javafx.event.EventUtil.fireEventImpl(EventUtil.java:74)
at com.sun.javafx.event.EventUtil.fireEvent(EventUtil.java:54)
at javafx.event.Event.fireEvent(Event.java:204)
at javafx.scene.Scene$KeyHandler.process(Scene.java:3949)
at javafx.scene.Scene$KeyHandler.access$2100(Scene.java:3896)
at javafx.scene.Scene.impl_processKeyEvent(Scene.java:2036)
at javafx.scene.Scene$ScenePeerListener.keyEvent(Scene.java:2493)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:170)
at com.sun.javafx.tk.quantum.GlassViewEventHandler$KeyEventNotification.run(GlassViewEventHandler.java:123)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.tk.quantum.GlassViewEventHandler.handleKeyEvent(GlassViewEventHandler.java:197)
at com.sun.glass.ui.View.handleKeyEvent(View.java:517)
at com.sun.glass.ui.View.notifyKey(View.java:927)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$300(WinApplication.java:39)
at com.sun.glass.ui.win.WinApplication$4$1.run(WinApplication.java:112)
at java.lang.Thread.run(Thread.java:744)