To reproduce type several words separated by space in the text area. Put caret to the beginning and using Alt + Shift + Left Arrow start selecting word after word. You will get

Exception in thread "JavaFX Application Thread" java.lang.StringIndexOutOfBoundsException: String index out of range: 23
at java.lang.String.charAt(String.java:651)
at javafx.scene.control.TextInputControl.endOfNextWord(TextInputControl.java:679)
at javafx.scene.control.TextInputControl.selectEndOfNextWord(TextInputControl.java:595)
at com.sun.javafx.scene.control.behavior.TextInputControlBehavior.selectNextWord(TextInputControlBehavior.java:440)
at com.sun.javafx.scene.control.behavior.TextInputControlBehavior.selectRightWord(TextInputControlBehavior.java:458)
at com.sun.javafx.scene.control.behavior.TextInputControlBehavior.callAction(TextInputControlBehavior.java:178)
at com.sun.javafx.scene.control.behavior.TextAreaBehavior.callAction(TextAreaBehavior.java:265)
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)