public boolean removeAction(@Nullable Action a) {
        if (a == null) {
            return false;
        }
        // CopyOnWriteArrayList does not support Iterator.remove, so need to do it this way:
        return getActions().removeAll(Set.of(a));
    }
@Test
    public void removeAction_null() {
        assertFalse(thing.removeAction(null));
    }