TextFieldXXCell: NPE on calling startEdit

happens if XX is null - as there is no pre-condition for calling the method, it must accept the state (the base implementations do, but not the TextField variants, didn't test any of the others like combo, picker ..)

failing test for ListCell, for comparison the passing base:

    /**
     * NPE on all TextFieldXXCell (not with base XXCell!)
     */
    @Test
    public void testTextFieldCellNullControlOnStartEditStandalone() {
        ListCell cell = TextFieldListCell.forListView().call(null);
        cell.startEdit();
    }

    /**
     * NPE on all TextFieldXXCell (not with base XXCell!)
     */
    @Test
    public void testBaseNullControlOnStartEditStandalone() {
        ListCell cell = new ListCell();
        cell.startEdit();
    }