RadioMenuItem in ToggleGroup: deselected on accelerator

Though RadioMenuItems in a ToggleGroup cannot be deselected by mouse, they can be deselected by accelerator.

Steps:
* in the attached sample application, press twice "Shortcut+1" -> the first item will get selected and then deselected again


Expected: consistent behavior

I think RadioMenuItem should neither be deselectable by mouse nor by accelerator, but maybe this could be made configurable.