The save method that triggers the green alert box.

LeftAndMain.php

<?php
   // ...


   /**
     * Save  handler
     *
     * @param array $data
     * @param Form $form
     * @return HTTPResponse
     */
    public function save($data, $form)
    {
        $request = $this->getRequest();
        $className = $this->config()->get('tree_class');
        // Existing or new record?
        $id = $data['ID'];
        if (is_numeric($id) && $id > 0) {
            $record = DataObject::get_by_id($className, $id);
            if ($record && !$record->canEdit()) {
                return Security::permissionFailure($this);
            }
            if (!$record || !$record->ID) {
                $this->httpError(404, "Bad record ID #" . (int)$id);
            }
        } else {
            if (!singleton($this->config()->get('tree_class'))->canCreate()) {
                return Security::permissionFailure($this);
            }
            $record = $this->getNewItem($id, false);
        }
        // save form data into record
        $form->saveInto($record, true);
        $record->write();
        $this->extend('onAfterSave', $record);
        $this->setCurrentPageID($record->ID);
        $message = _t(__CLASS__ . '.SAVEDUP', 'Saved.');
        if ($this->getSchemaRequested()) {
            $schemaId = Controller::join_links($this->Link('schema/DetailEditForm'), $id);
            // Ensure that newly created records have all their data loaded back into the form.
            $form->loadDataFrom($record);
            $form->setMessage($message, 'good');
            $response = $this->getSchemaResponse($schemaId, $form);
        } else {
            $response = $this->getResponseNegotiator()->respond($request);
        }
        $response->addHeader('X-Status', rawurlencode($message));
        return $response;
    }

    // ...