How to create a custom field programmatically on content type creation?
It is the NodeType::load() in seo_form_submit() that is failing because the bundle doesn't exist at the time the form is submitted for the first time.

My recommendation would be to redesign this code so that the logic to create/destroy the field is in both a hook_node_type_insert() and a hook_node_type_update().

You will need some way to pass the value of the checkbox on the form to these respective hooks. Most likely through a custom configuration or schema.

Edit: It looks like there is more than one design pattern that would work as well as the one I recommended in this answer. Please reference Entity CRUD, editing, and view hooks.