public function getPrototypeWidget($column_id)
    {
        if (isset($this->input_cells[$column_id])) {
            return $this->input_cells[$column_id]->getPrototypeWidget();
        }

        throw new SwatException(
            'The specified column does not have an input ' .
                'cell bound to this row or the column does not exist.'
        );
    }