overriding GridFieldAddNewMultiClass ItemRequestClass to set some default ManyMany values

gistfile1.php

<?php

class GridFieldConfig_FlexiForm extends GridFieldConfig {

    public static function include_requirements() {
        $moduleDir = self::get_module_dir();
        Requirements::css($moduleDir.'/css/flexiforms.css');
    }

    public static function get_module_dir() {
        return basename(dirname(__DIR__));
    }


    public function __construct() {
        $this->addComponent(new GridFieldButtonRow('before'));
        $this->addComponent(new GridFieldAddNewMultiClass('buttons-before-left'));
        $this->addComponent(new GridFieldAddExistingAutocompleter('buttons-before-right'));
        $this->addComponent(new GridFieldToolbarHeader());
        $this->addComponent(new GridFieldTitleHeader());
        $this->addComponent(new GridFieldEditableColumns());
        $this->addComponent(new GridFieldEditButton());
        $this->addComponent(new GridFieldDeleteAction(true));
        $this->addComponent(new GridFieldDetailForm());

        $component = $this->getComponentByType('GridFieldAddNewMultiClass');
        $component->setItemRequestClass('GridFieldDetailForm_FlexiFormRequest');

        self::include_requirements();
    }

}


class GridFieldDetailForm_FlexiFormRequest extends GridFieldAddNewMultiClassHandler {

    public function doSave($data, $form) {
        $new_record = $this->record->ID == 0;

        if($new_record) {
           $data['ManyMany'] = array(
               'Name' => $data['FieldName'],
               'DefaultValue' => $data['FieldDefaultValue']
           );
        }

        parent::doSave($data,$form);
    }

}


....

class FlexiForm extends Page
{

    protected $flexiform_tab = 'Root.Form';

    private static $db = array();



    private static $has_many = array(
        'Submissions'
    );

    private static $many_many = array(
        'Fields' => 'FlexiFormField'
    );

    private static $many_many_extraFields = array(
        'Fields' => array(
            'Name' => 'Varchar',
            'Prompt' => 'Varchar',
            'DefaultValue' => 'Varchar',
            'Required' => 'Boolean',
            'SortOrder' => 'Int'
        )
    );

    protected $allowed_field_types = array(
        'FlexiFormTextField',
        'FlexiFormEmailField',
        'FlexiFormDropdownField',
        'FlexiFormCheckboxField',
        'FlexiFormRadioField',
        'FlexiFormCheckboxSetField'
    );

    ...