Undefined variable and index using shortcode

I’m using the shortcode function and I’m getting this message initially:

Notice: Undefined variable: instance in ../dotmailer-sign-up-widget/dm_shortcode.php on line 16

And after submitting the form, I get another error message, as well as the previous one:

Notice: Undefined index: page in ../dotmailer-sign-up-widget/vendor/romanpitak/dotmailer-api-v2-client/src/Rest/Client.php on line 32

Has anyone got any idea of how to get rid of these messages? Without setting WP_DEBUG to false 🙂

https://ww.wp.xz.cn/plugins/dotmailer-sign-up-widget/