The offset value notice did appear as long as the field “banner_image” is still empty.
My solution above was just to stop the notice.
Correct it should be like this, maybe somebody has a better way:
if (isset(get_field('banner_image', get_option('page_for_posts'))['url'])) {
$image = get_field('banner_image', get_option('page_for_posts'))['url'];
}
if(empty($image)){
$image = get_template_directory_uri().'/assets/img/breadcrumb/breadcrumb-bg-1.jpg';
}