Error showing in current page id in wordpress admin

Question

I am trying to remove a metabox ID from a page ID in the admin screen. So I have coded the following:

function ag_remove_cmb_metaboxes() {
    global $post;
    $id = $post->ID;
    if ($id == 21) { ?>
        <script type="text/javascript">
        jQuery(document).ready(function($){
                jQuery('#test_metabox_img') .hide();
        });
        </script>
    <?php }
}
add_action( 'admin_head', 'ag_remove_cmb_metaboxes' );

It is working fine but an error is showing in the widget.php page. Error code is:

Notice: Trying to get property of non-object in E:\xampp\htdocs\speechcare\wp-content\themes\cutedoctor\core\helpers.php on line 56

Can anyone please explain why this error is showing and what I am doing wrong? Thanks in advance.

Answer 1

While front–end of the site always has a post context (you are either looking at one or archive of posts), admin side doesn't.

So plenty of admin screens won't have $post global set to anything. You should be checking if it's meaningful (for example by if ( ! empty( $post ) )) before trying to access it.