As Sally CJ suggest in comments, the error come from the terms with artist_avatar
empty. So, check if is_array()
before access the array, fix it:
add_filter('manage_artist_custom_column', function ($content,$column_name,$term_id){
$term = get_term($term_id, 'artist');
$avatar = get_field('artist_avatar', $term);
switch ($column_name) {
case 'artist_avatar':
if ( is_array( $avatar ) ) {
$content = $avatar["url"];
} else {
$content = __('This artist has no picture', 'sage');
}
break;
default:
break;
}
return $content;
},10,3);