Undefined index: add_to_cart_bookable

I have tons of this error in the php error logs, can be fixed?
public_html/wp-content/plugins/add-to-cart-button-custom-text/class.php on line 142
I did that to silence the error:

if ($this->ajustes = get_option ('add_to_cart_button_text_settings'))
				foreach ($opciones as $opcion => $default)
                    if ( isset( $this->ajustes[$opcion] ) ) {
                        $this->$opcion = $this->ajustes[$opcion];
					}