FIX: Use Hierarchy::prepopulate_numchildren_cache in tree-generation

Only relevant if silverstripe/silverstripe-framework#8380 is avialable,
however coded defensively so it can be merged before that PR if needs 
be.

See silverstripe/silverstripe-framework#8379

code/Controllers/CMSMain.php

@@ -54,6 +54,7 @@
 use SilverStripe\ORM\DB;
 use SilverStripe\ORM\FieldType\DBHTMLText;
 use SilverStripe\ORM\HiddenClass;
+use SilverStripe\ORM\Hierarchy\Hierarchy;
 use SilverStripe\ORM\Hierarchy\MarkedSet;
 use SilverStripe\ORM\SS_List;
 use SilverStripe\ORM\ValidationResult;
@@ -493,6 +494,11 @@ public function SiteTreeAsUL()
         // Pre-cache sitetree version numbers for querying efficiency
         Versioned::prepopulate_versionnumber_cache(SiteTree::class, Versioned::DRAFT);
         Versioned::prepopulate_versionnumber_cache(SiteTree::class, Versioned::LIVE);
+
+        if (method_exists(Hierarchy::class, 'prepopulate_numchildren_cache')) {
+            Hierarchy::prepopulate_numchildren_cache(SiteTree::class, Versioned::DRAFT);
+        }
+
         $html = $this->getSiteTreeFor($this->config()->get('tree_class'));
 
         $this->extend('updateSiteTreeAsUL', $html);