SilverStripe subsite tree drop down

Page.php

<?php
        $subsites = DataObject::get(Subsite::class)->map('ID', 'Title');

        $fields->insertBefore('internal_linkID',
            DropdownField::create('internal_linkID_SubsiteID', 'Subsite')
                ->setSource($subsites->map('ID', 'Title'))
                ->addExtraClass('subsitestreedropdownfield-chooser no-change-track')
        );

        $internalLinkField = SubsitesTreeDropdownField::create(
          'internal_linkID',
          'Internal link',
          SiteTree::class,
          'ID',
          'MenuTitle'
        )->setDescription('If provided, overrides the external link.');

        if (Controller::has_curr() && Controller::curr()->getRequest()) {
            $internalLinkField->setSubsiteID(
                (int) Controller::curr()->getRequest()->requestVar('internal_linkID_SubsiteID')
            );
        }

        // In this instance, the internal_linkID already exists as a TreeDropdownField
        $fields->replaceField('internal_linkID', $internalLinkField);