I have solved the problem by loading select2.full in "select-advanced.php" file
[code]
// wp_register_script( 'rwmb-select2', RWMB_JS_URL . 'select2/select2.min.js', array( 'jquery' ), '4.0.10', true );
wp_register_script( 'rwmb-select2', 'https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.13/js/select2.full.js', array( 'jquery' ), '4.0.13', true );
[/code]