IllegalStateException on an observable list

[Tom Eugelink]

I have two observable lists; one containing appointments, the other containing selected appointments. If an appointment is removed from the first list, it also must be removed from the second. For this I setup a listener on the first list to handle this:

         // when appointments are removed, they can't be selected anymore
         appointments.addListener(new ListChangeListener<Agenda.Appointment>()
         {
             @Override
             public void onChanged(javafx.collections.ListChangeListener.Change<? extends Appointment> changes)
             {
                 for (Appointment lAppointment : changes.getRemoved())
                 {
                     selectedAppointments.remove(lAppointment);
                 }
             }
         });

when constructing the control I get the following exception on the getRemoved() call.

Caused by: java.lang.IllegalStateException
     at com.sun.javafx.collections.NonIterableChange.checkState(NonIterableChange.java:101)
     at com.sun.javafx.collections.NonIterableChange$SimpleAddChange.getRemoved(NonIterableChange.java:158)
     at jfxtras.labs.scene.control.Agenda$1.onChanged(Agenda.java:108)
     at com.sun.javafx.collections.ListListenerHelper$SingleChange.fireValueChangedEvent(ListListenerHelper.java:134)
     at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:48)
     at com.sun.javafx.collections.ObservableListWrapper.callObservers(ObservableListWrapper.java:97)
     at com.sun.javafx.collections.ObservableListWrapper.addAll(ObservableListWrapper.java:171)
     at com.sun.javafx.collections.ObservableListWrapper.addAll(ObservableListWrapper.java:160)
     at com.sun.javafx.collections.ObservableListWrapper.addAll(ObservableListWrapper.java:309)
     at jfxtras.labs.scene.control.AgendaTrial1.start(AgendaTrial1.java:122)
     at com.sun.javafx.application.LauncherImpl$5.run(LauncherImpl.java:319)
     at com.sun.javafx.application.PlatformImpl$5.run(PlatformImpl.java:206)
     at com.sun.javafx.application.PlatformImpl$4.run(PlatformImpl.java:173)
     at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
     at com.sun.glass.ui.win.WinApplication.access$100(WinApplication.java:29)
     at com.sun.glass.ui.win.WinApplication$2$1.run(WinApplication.java:62)
     ... 1 more

Can someone explain why? And how to work around that?

Tom