Error with non-existing array offset

[jhtjards]

Hi, thanks for creating this Plugin!

I've come across an issue where the plugin triggered a fatal error in class-sippis-acf-field-network-post-select.php:356

I'm not 100% sure how it was triggered, maybe by deleting a previously selected post, or maybe by saving an empty field, but I've fixed this by expanding the isset check to all used offsets in line 356:

$field['value'] = isset($field['value'], $field['value']['site_id'], $field['value']['post_id']) ? $field['value']['site_id'] . '|' . $field['value']['post_id'] : '';

Feel free to incorporate it.

[rollecode]

@jhtjards Please also note that we're continuing the development here (moving so fast that decided just to go on with the fork). This fix and some more exist in our version.