[6.0] Update Site ID fatal error (joomla#45215)

Another fix as a result of "return a stdClass instead of CMSObject" joomla#42961

Author: brianteeman

administrator/components/com_installer/src/View/Updatesite/HtmlView.php

@@ -70,7 +70,7 @@ public function display($tpl = null): void
 
         // Remove the extra_query field if it's a free download extension
         $dlidSupportingSites = InstallerHelper::getDownloadKeySupportedSites(false);
-        $update_site_id      = $this->item->get('update_site_id');
+        $update_site_id      = $this->item->update_site_id;
 
         if (!\in_array($update_site_id, $dlidSupportingSites)) {
             $this->form->removeField('extra_query');