我正在尝试做一个程序，当一个项目与相同的描述，标题，颜色等创建它将改变旧的价格。

但是，当我在SQL表中创建一个新行时，会出现以下错误

Notice: Trying to access array offset on value of type null in C:\xampp\htdocs\Projects\dad\PHP\addproduct.php on line 18

下面是我到目前为止制作的代码。

<?php
    require("./sql.info.php");

    $title = stripcslashes(mysqli_real_escape_string($con, $_POST["Title"]));
    $col = stripcslashes(mysqli_real_escape_string($con, $_POST["ColourMat"]));
    $desc = stripcslashes(mysqli_real_escape_string($con, $_POST["Description"]));
    $price = stripcslashes(mysqli_real_escape_string($con, $_POST["Price"]));

    $title_low = strtolower($title);
    $col_low = strtolower($col);
    $desc_low = strtolower($desc);

    if (isset($_POST["Submit"])) {
        if (isset($title) && isset($title_low) && isset($col) && isset($col_low) && isset($desc) && isset($desc_low) && isset($price)) {
            $res = mysqli_query($con, "SELECT * FROM products WHERE lower(Title) = '$title_low' AND lower(ColourMaterial) = '$col_low' AND lower(ItemDescription) = '$desc_low'") or die(mysqli_error($con));
            $row = mysqli_fetch_array($res);

            if (strtolower($row[1]) == $title_low && strtolower($row[2]) == $col_low && strtolower($row[3]) == $desc_low) {
                $query = mysqli_query($con, "REPLACE INTO products VALUES ('$row[0]', '$title', '$col', '$desc', '$price')");
                header("Location: ../a/test.html");
            } else {
                $query = mysqli_query($con, "INSERT INTO products (Title, ColourMaterial, ItemDescription, Price) VALUES ('$title', '$col', '$desc', '$price')");
            }
        }
    }
?>