Warning: Trying to access array offset on value of type null 

I´m getting this warning in my ChatApp userlist. The ChatApp is made with php and mysql.

This is the line were the warning appears:

($outgoing_id == $row2['outgoing_msg_id']) ? $you = "You: " : $you = "";

This is the whole data.php file:

<?php 
    while($row = mysqli_fetch_assoc($sql)){
        $sql2 = "SELECT * FROM messages WHERE (incoming_msg_id = {$row['unique_id']}
                 OR outgoing_msg_id = {$row['unique_id']}) AND (outgoing_msg_id = {$outgoing_id}
                 OR incoming_msg_id = {$outgoing_id}) ORDER BY msg_id DESC LIMIT 1";
        $query2 = mysqli_query($conn, $sql2);
        $row2 = mysqli_fetch_assoc($query2);
        if(mysqli_num_rows($query2) > 0){
            $result = $row2['msg'];
        }else{
            $result = "No messages avaiable";
        }

        (strlen($result) > 28) ? $msg = substr($result, 0, 28).'...' : $msg = $result;
        ($outgoing_id == $row2['outgoing_msg_id']) ? $you = "You: " : $you = "";

        $output .= '<a href="chat.php?user_id='.$row['unique_id'].'">
                    <div class="content">
                    <img src="php/images/'. $row['img'] .'" alt="">
                    <div class="details">
                        <span>'. $row['fname'] . " " . $row['lname'] .'</span>
                        <p>'. $you . $msg .'</p>
                    </div>
                    </div>
                    <div class="status-dot"><i class="fas fa-circle"></i></div>
                    </a>';
    }
?>

The whole ChatApp runs with XAMPP on localhost

We don´t know what we could do.