Fields are empty when using a form to edit a post

If you find the form fields to be empty when editing an existing post and you notice that submitting the form results in new posts being created instead of the desired post being updated, this means the form isn't currently configured correctly to edit a post and instead is defaulting to creating new posts.

To fix this, check to see if a `post` argument has been set either on the shortcode or in the array passed to the advanced_form() function. e.g; 

To edit the current post:

[advanced_form form="KEY" post="current"]

[advanced_form form="KEY" post="123"]

advanced_form( 'FORM_KEY', [ 'post' => 123 ]);