main(): Couldn't fetch mysqli_stmt in line 31 if ($stmt->num_rows > 0)

Question

<?php
session_start();
// Change this to your connection info.

$DATABASE_HOST = 'localhost';
$DATABASE_USER = 'root';
$DATABASE_PASS = '';
$DATABASE_NAME = 'phplogin';

// Try and connect using the info above.
$con = mysqli_connect($DATABASE_HOST, $DATABASE_USER, $DATABASE_PASS, $DATABASE_NAME);
if ( mysqli_connect_errno() ) {
    // If there is an error with the connection, stop the script and display the error.
    exit('Failed to connect to MySQL: ' . mysqli_connect_error());
}
// Now we check if the data from the login form was submitted, isset() will check if the data exists.
if ( !isset($_POST['username'], $_POST['password']) ) {
    // Could not get the data that should have been sent.
    exit('Please fill both the username and password fields!');
}
// Prepare our SQL, preparing the SQL statement will prevent SQL injection.
if ($stmt = $con->prepare('SELECT id, password FROM accounts WHERE username = ?')) {
    // Bind parameters (s = string, i = int, b = blob, etc), in our case the username is a string so we use "s"
    $stmt->bind_param('s', $_POST['username']);
    $stmt->execute();
    // Store the result so we can check if the account exists in the database.
    $stmt->store_result();
    $stmt->close();
}

if ($stmt->num_rows > 0) {
    $stmt->bind_result($id, $password);
    $stmt->fetch();
    // Account exists, now we verify the password.
    // Note: remember to use password_hash in your registration file to store the hashed passwords.
    if ($_POST['password'] === $password) {
        // Verification success! User has loggedin!
        // Create sessions so we know the user is logged in, they basically act like cookies but remember the data on the server.
        session_regenerate_id();
        $_SESSION['loggedin'] = TRUE;
        $_SESSION['name'] = $_POST['username'];
        $_SESSION['id'] = $id;
        echo header('Location: home.php');
    } else {
        echo 'Incorrect password!';
    }
} else {
    echo 'Incorrect username!';
}
?>