Redeclare function after plugin activation

Ok with helps it’s working. So theme was ok in front of wordpress but wasn’t able to load in the admin so we changed the previous condition by :

if (!function_exists('get_field') && !is_admin()) {
    function get_field()
    {
        return;
    }
    function the_field()
    {
        return;
    }
}

but same problem with activation in the admin and it was because I use acf in my functions.php too.

if (!empty(get_post(get_field('login', 'options')))) {

changed in :

if( get_option('login') ) {