Running WP_Query causing issue

Hi,

In my wordpress page, i added a form where i am getting a serial number, and this number i am matching with the custom fields and retrieving the posts. However when i click on the button, it is crashing to error page. Please check what is the issue. Somewhere it says when we put code in snippets, we have to initialize WP_Query.

function display_search_book () {
 return "
	<form method='post'>
	Enter book serial number: 
	<input type='text' required name='search_book' />
	<input type='submit' name='book_search' value='Add Book' />
	</form>
	<p id='show_message'></p>
	";
}
add_shortcode('show_num', 'display_search_book');

$message="";
if (isset($_POST['book_search'])) {
$book_serial=$_POST['search_book'];
$userid=get_current_user_id();
global $wpdb;
$query = new WP_Query( array( 'post_type' => '3d-flip-book', 'meta_key' => 'serial_number', 'meta_value' => $book_serial ) );
if( $query->have_posts() ) {
    while( $query->have_posts() ) { 
        $query->the_post();

        $que=$wpdb->insert('sk_book_addition', array(
                    'book_id' => $post->ID,
                    'user_id' => $userid,
                   )
         );
        if($que){$message.="Book Added !!";}
        else{$message.="No Book Added !!";}

    }
}
else{
    $message.="No Books Found.";
}
}

function display_messages () {
return "

".$message."";
}
add_shortcode('show_messages', 'display_messages');

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