Wordpress Custom taxonomy dropdown list array to tagdiv composer - alphabetical

Question

I am currently working on a new wordpress website which uses the Newspaper theme, which lets me compose pages with the tagdiv composer plugin.
This plugin makes it fairly easy to show posts of your choosing; with blocks/modules that have built in filters, that allow you to select the posts to be shown based upon the categories, authors, etc. that you choose.
However I would like to show custom taxonomies in there as well, so that I can choose for instance album reviews, which have the custom taxonomy of the letter A attached to it.
For my client I would need to do that since he requested a glossary (alphabetical index).
So far I have created a custom taxonomy called "alphabetical_letter", which houses all 26 characters with the linked posts to it.
The code which I need to alter is this one, preferrably I'd have a drop down list. However I keep failing in getting it right. Could anyone point me in the right direction please ?
/**
* the filter array (used by blocks and by the loop filters)
* #return array
*/
static function get_map_filter_array ($group = 'Filter') {
return array(
array(
"param_name" => "separator",
"type" => "text_separator",
'heading' => 'Filters',
"value" => "",
"class" => "",
'group' => $group
),
array(
"param_name" => "post_ids",
"type" => "textfield",
"value" => '',
"heading" => 'Post ID filter:',
"description" => "Filter multiple posts by ID. Enter here the post IDs separated by commas (ex: 10,27,233). To exclude posts from this block add them with '-' (ex: -7, -16)",
"holder" => "div",
"class" => "tdc-textfield-big",
'group' => $group
),
array(
"param_name" => "category_id",
"type" => "dropdown",
"value" => td_util::get_category2id_array(),
"heading" => 'Category filter:',
"description" => "A single category filter. If you want to filter multiple categories, use the 'Multiple categories filter' and leave this to default",
"holder" => "div",
"class" => "tdc-dropdown-big",
'group' => $group
),
array(
"param_name" => "category_ids",
"type" => "textfield",
"value" => '',
"heading" => 'Multiple categories filter:',
"description" => "Filter multiple categories by ID. Enter here the category IDs separated by commas (ex: 13,23,18). To exclude categories from this block add them with '-' (ex: -9, -10)",
"holder" => "div",
"class" => "tdc-textfield-big",
'group' => $group
),
array(
"param_name" => "alphabetical_letter_id",
"type" => "dropdown",
"value" => td_util::get_alphabetical_letter_id_array(),
"heading" => 'Custom taxonomy:',
"description" => "alphabetical letter custom taxonomy",
"holder" => "div",
"class" => "tdc-dropdown-big",
'group' => $group
),
array(
"param_name" => "tag_slug",
"type" => "textfield",
"value" => '',
"heading" => 'Filter by tag slug:',
"description" => "To filter multiple tag slugs, enter here the tag slugs separated by commas (ex: tag1,tag2,tag3)",
"holder" => "div",
"class" => "tdc-textfield-big",
'group' => $group
),`enter code here`
Edit:
in the meantime I have tried to figure out the td_util I keep seeing.
There is a file called td_util.php one folder up from where this file is, and it mentions the category2id_array so I am currently trying to find out if the problem lies here.
for completeness, this is the bit of text that refers to the category2id array function above (or the other way around more likely).
/**
* generates a category tree, only on /wp_admin/, uses a buffer
* #param bool $add_all_category = if true ads - All categories - at the begining of the list (used for dropdowns)
* #return array
*/
private static $td_category2id_array_walker_buffer = array();
static function get_category2id_array($add_all_category = true) {
if (is_admin() === false) {
return array();
}
if (empty(self::$td_category2id_array_walker_buffer)) {
$categories = get_categories(array(
'hide_empty' => 0,
'number' => 1500
));
$td_category2id_array_walker = new td_category2id_array_walker;
$td_category2id_array_walker->walk($categories, 4);
self::$td_category2id_array_walker_buffer = $td_category2id_array_walker->td_array_buffer;
}
if ($add_all_category === true) {
$categories_buffer['- All categories -'] = '';
return array_merge(
$categories_buffer,
self::$td_category2id_array_walker_buffer
);
} else {
return self::$td_category2id_array_walker_buffer;
}
}
And a bit further down the file it pops up one last time
class td_category2id_array_walker extends Walker {
var $tree_type = 'category';
var $db_fields = array ('parent' => 'parent', 'id' => 'term_id');
var $td_array_buffer = array();
function start_lvl( &$output, $depth = 0, $args = array() ) {
}
function end_lvl( &$output, $depth = 0, $args = array() ) {
}
function start_el( &$output, $category, $depth = 0, $args = array(), $id = 0 ) {
$this->td_array_buffer[str_repeat(' - ', $depth) . $category->name . ' - [ id: ' . $category->term_id . ' ]' ] = $category->term_id;
}
function end_el( &$output, $page, $depth = 0, $args = array() ) {
}
}