@SuppressWarnings("ConstantConditions")
    public void addAction(@NonNull Action a) {
        if (a == null) {
            throw new IllegalArgumentException("Action must be non-null");
        }
        getActions().add(a);
    }
@Test
    public void addAction_null() {
        assertThrows(IllegalArgumentException.class, () -> thing.addAction(null));
    }