Trying to get property of non-object in php/ Creating default object from empty value in php

Question

As a newbie in PHP, I tried to make my own site and faced 2 errors, here's the code:

<?php
$servername = "localhost";
$username = "user";
$password = "pass";
$dbname = "db name";

$conn = new mysqli($servername, $username, $password, $dbname);
$num = $_GET['id'];
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
} 

$sql = "SELECT date, point, reason, teacher, giver FROM penalty WHERE studentid=$num";
$result = $conn->query($sql);

if ($result->num_rows > 0) 
{   
    echo "<table>";
    echo "<tr>";
    echo "<td>date</td>";
    echo "<td>score</td>";
    echo "<td>reason</td>";
    echo "<td>giver</td>";
    echo "<td>teacher</td>";
    echo "</tr>";
    while($row = $result->fetch_assoc()) 
    {
        echo "<tr>";
        echo "<td>".$row["dated"]."</td>";
        echo "<td>".$row["point"]."</td>";
        echo "<td>".$row["reason"]."</td>";
        echo "<td>".$row["giver"]."</td>";
        echo "<td>".$row["teacher"]."</td>";
        echo "</tr>";
    }
    echo "</table>";
}

else if ($result->num_rows = 0)
{
    echo "0!";
}

else 
{
    echo "empty";
}

$conn->close();
?>

and the results:

Notice: Trying to get property of non-object in view.php on line 16 Warning: Creating default object from empty value in view.php on line 39 empty

What can be the problem to cause this error, and how should I fix it?

Thanks in advance