Estou implementando drag'n'drop para visualizações. Quando o arrasto é iniciado, defino a visibilidade da exibição paraINVISIBLE, então, se o arrasto foi interrompido - de volta paraVISIBLE:

public boolean onTouch(View v, MotionEvent event) {
    if (event.getAction() == MotionEvent.ACTION_DOWN) {
        // Skipped some code
        boolean dragStarted = v.startDrag(data, shadowBuilder, v, 0);

        if (dragStarted) {
            v.setVisibility(View.INVISIBLE)
        }
    }
}

E:

if (event.getAction() == DragEvent.ACTION_DRAG_ENDED) {
    View droppedView = (View) event.getLocalState();
    droppedView.setVisibility(View.VISIBLE);
}

E quando o evento "Drag ended" é chamado, estou recebendo uma exceção:

E/AndroidRuntime(7118): FATAL EXCEPTION: main 
E/AndroidRuntime(7118): java.util.ConcurrentModificationException 
E/AndroidRuntime(7118):     at java.util.HashMap$HashIterator.nextEntry(HashMap.java:792)
E/AndroidRuntime(7118):     at java.util.HashMap$KeyIterator.next(HashMap.java:819) 
E/AndroidRuntime(7118):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1046)
E/AndroidRuntime(7118):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1048)
E/AndroidRuntime(7118):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1048)
E/AndroidRuntime(7118):     at android.view.ViewGroup.dispatchDragEvent(ViewGroup.java:1048)
E/AndroidRuntime(7118):     at android.view.ViewRootImpl.handleDragEvent(ViewRootImpl.java:3471)
E/AndroidRuntime(7118):     at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2620)
E/AndroidRuntime(7118):     at android.os.Handler.dispatchMessage(Handler.java:99) 
E/AndroidRuntime(7118):     at android.os.Looper.loop(Looper.java:137) 
E/AndroidRuntime(7118):     at android.app.ActivityThread.main(ActivityThread.java:4575)
E/AndroidRuntime(7118):     at java.lang.reflect.Method.invokeNative(Native Method) 
E/AndroidRuntime(7118):     at java.lang.reflect.Method.invoke(Method.java:511)
E/AndroidRuntime(7118):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
E/AndroidRuntime(7118):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
E/AndroidRuntime(7118):     at dalvik.system.NativeStart.main(NativeMethod)

Por que e como consertar isso?