Plugin development – Database error (Fatal error: Call to a member function..)

Hello WordPress community,
I’m currently developing Star Rating plugin and I get the following error:
“
<b>Fatal error</b>: Call to a member function fetch_assoc() on a non-object in <b>C:\Users\User\Dropbox\XAMPP\htdocs\events\wp-content\plugins\review-plugin\inc\data-processing.php</b> on line <b>45</b>”
when I try to save data in database.

I’m new in plugin development, please help me resolve this issue.

Thanks!

Here is my front-end code:

$post_rating = "
			 <fieldset id='demo1' class='rating'>
                        <input class='stars' type='radio' id='star5' name='rating' value='5' />
                        <label class = 'full' for='star5' title='Awesome - 5 stars'></label>
                        <input class='stars' type='radio' id='star4' name='rating' value='4' />
                        <label class = 'full' for='star4' title='Pretty good - 4 stars'></label>
                        <input class='stars' type='radio' id='star3' name='rating' value='3' />
                        <label class = 'full' for='star3' title='Meh - 3 stars'></label>
                        <input class='stars' type='radio' id='star2' name='rating' value='2' />
                        <label class = 'full' for='star2' title='Kinda bad - 2 stars'></label>
                        <input class='stars' type='radio' id='star1' name='rating' value='1' />
                        <label class = 'full' for='star1' title='Sucks big time - 1 star'></label>

                    </fieldset>

                  <script src='https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js'></script>
                    <script>
					    $('.stars').click(function () {

					        $.post('localhost/events/wp-content/plugins/review-plugin/inc/data-processing.php',{rate:$(this).val()},function(d){
					        	console.log(d, 'here');
					            if(d>0)
					            {
					                alert('You already rated');
					            }else{
					                alert('Thanks For Rating');
					            }
					        });
					        $(this).attr('checked');
					    });
                    </script>

		";

(jQuery is here because I’m testing)

Saving in database code (data-proccesing.php) :

<?php
$ipaddress = md5($_SERVER['REMOTE_ADDR']); // here I am taking IP as UniqueID but you can have user_id from Database or SESSION

$servername = "localhost"; // Server details
$username = "root";
$password = "";
$dbname = "events";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("Unable to connect Server: " . $conn->connect_error);
}

if (isset($_POST['rate']) && !empty($_POST['rate'])) {

    $rate = $conn->real_escape_string($_POST['rate']);
// check if user has already rated
    $sql = "SELECT <code>id</code> FROM <code>tbl_rating</code> WHERE <code>user_id</code>='" . $ipaddress . "'";
    $result = $conn->query($sql);
    $row = $result->fetch_assoc();
    if ($result->num_rows > 0) {
        echo $row['id'];
    } else {

        $sql = "INSERT INTO <code>tbl_rating</code> ( <code>rate</code>, <code>user_id</code>) VALUES ('" . $rate . "', '" . $ipaddress . "'); ";
        if (mysqli_query($conn, $sql)) {
            echo "0";
        }
    }
}
$conn->close();
?>