csstidy warning

Trying to access array offset on value of type int
/wp-content/plugins/bm-custom-login/csstidy/class.csstidy_optimise.php — line 599
/wp-content/plugins/bm-custom-login/csstidy/class.csstidy_optimise.php — line 88
/wp-content/plugins/bm-custom-login/csstidy/class.csstidy.php — line 934
/wp-content/plugins/bm-custom-login/bm-custom-login.php — line 241
/wp-content/plugins/bm-custom-login/bm-custom-login.php — line 208
/wp-includes/class-wp-hook.php — line 324
/wp-includes/class-wp-hook.php — line 348
/wp-includes/plugin.php — line 517
/wp-login.php — line 127
/wp-login.php — line 1482

FIX:

static function merge_4value_shorthands($array) {
    $return = $array;
    $shorthands = &$GLOBALS['csstidy']['shorthands'];

    foreach ($shorthands as $key => $value) {
        // Check if $value is an array and has at least 4 elements
        if (is_array($value) && count($value) >= 4) {
            if (isset($array[$value[0]]) && isset($array[$value[1]]) && 
                isset($array[$value[2]]) && isset($array[$value[3]]) && $value !== 0) {
                
                $return[$key] = '';

                $important = '';
                for ($i = 0; $i < 4; $i++) {
                    $val = $array[$value[$i]];
                    if (csstidy::is_important($val)) {
                        $important = '!important';
                        $return[$key] .= csstidy::gvw_important($val) . ' ';
                    } else {
                        $return[$key] .= $val . ' ';
                    }
                    unset($return[$value[$i]]);
                }
                $return[$key] = csstidy_optimise::shorthand(trim($return[$key] . $important));
            }
        }
    }
    return $return;
}