Minor Issue (PHP Notice for Undefined Index)

Hello,
First of all I would like to mention that the plugin is great! 😀

I would like to report a minor issue where a notice is rised from the code. The problematic file is /admin/metabox.php on line 99, in the if statement.

The notice is regular for undefined index, which might be fixed with isset check, like (if statement is skipped to be more clear):

is_array( $vars ) && isset( $vars['active'] ) && $vars['active']

That is the only one I have encountered to the current moment, which means the code is well written and in case I find any other I will write here.

Thanks

https://wordpress.org/plugins/multiple-gallery-on-post/