Hi,

Maybe there is a item of your slider that is image but you did not select image or image does not exit so error displays. Please re-upload image or go to file md_slider/templates/admin/theme.admin.inc, replace lines from 68 to 76 with:

if ($layer['type'] == 'image') {
$layer['file_url'] = '';
if (isset($layer['fileid']) && $layer['fileid'] > 0) {
$image = file_load($layer['fileid']);
if (isset($image->uri)) {
$layer['file_url'] = file_create_url($image->uri);
}
}
}