Add taxonomy column with meta field (ACF) content

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);