CodeIgniter - Grocery CRUD：删除require上传文件

Question

I have an issue when delete the upload file on edit. If upload field is require, after delete this file I can’t save the record. But if I close browser and open again, this record has empty upload field without any error. Is there any solution? Thanks. EDIT: Source code is very simple... just 2 lines

$crud->required_fields ( 'name', 'image', 'price' );
$crud->set_field_upload ( 'image', $this->config->item ( 'image_path' ) );