Use Current Post ID as a QUERY ARG

I still can't get this to work. Here's example code in my functions.php file.

function monitor_set_controller_number( $meta_boxes ) {
    
    $prefix = '';
    
    $post_id = null;
// Method #1 - DID NOT WORK ($post_id is NULL)
//    if ( isset( $_GET['post'] ) ) {
//  $post_id = intval( $_GET['post'] );
//    } elseif ( isset( $_POST['post_ID'] ) ) {
//        $post_id = intval( $_POST['post_ID'] );
//    }

// Method #2 - DID NOT WORK ($post_id is NULL)
//    $post_id = $GLOBALS['post']->ID;

// Method #3 - DID NOT WORK ($post_id is NULL)
//    $post_id = rwmb_meta( $field_id );

    $meta_boxes[] = [
        'title'        => __( 'Monitor Fields', 'your-text-domain' ),
        'id'           => 'monitor-fields',
        'post_types'   => ['monitor'],
        'storage_type' => 'custom_table',
        'table'        => 'monitor_table',
        'tabs'         => [
            'monitor_details'      => [
                'label' => 'Monitor Details',
                'icon'  => 'chart-bar',
            ],
            'tank_configuration'   => [
                'label' => 'Tank Configuration',
                'icon'  => 'category',
            ],
            'local_display_fields' => [
                'label' => 'Local Display Fields',
                'icon'  => 'money',
            ],
        ],
        'fields'       => [
            [
                'name'       => __( 'Location', 'your-text-domain' ),
                'id'         => $prefix . 'physical_location',
                'type'       => 'post',
                'post_type'  => ['location'],
                'field_type' => 'select',
                'order by'   => 'post_id',
                'tab'        => 'monitor_details',
            ],
            [
                'name'      => __( 'Active', 'your-text-domain' ),
                'id'        => $prefix . 'monitor_active',
                'type'      => 'switch',
                'style'     => 'square',
                'on_label'  => 'Active',
                'off_label' => 'Inactive',
                'std'       => true,
                'tab'       => 'monitor_details',
            ],
            [
                'name'       => __( 'Controller', 'your-text-domain' ),
                'id'         => $prefix . 'controller',
                'type'       => 'post',
                'post_type'  => ['controller'],
                'field_type' => 'select',
                'query_args' => [
                    'orderby'     => 'post_title',
                    'order'       => 'ASC',
                    'post_parent' => $post_id,
                ],
                'tab'        => 'monitor_details',
            ],
            :

It seems like the post doesn't exist at the time the functions.php is included so it's not defined.

I know it's reading in the functions.php file because I can change $post_id to 200017 and it brings up the correct controllers.

That didn't work.

I think the code is correct but I did change the post_types from "Monitor" to "Controller" in the first few lines of code to test.

    $meta_boxes[] = [
        'title'        => __( 'Monitor Fields', 'your-text-domain' ),
        'id'           => 'monitor-fields',
        'post_types'   => ['<strong>monitor</strong>'],
        'storage_type' => 'custom_table',
        'table'        => 'monitor_table',

I left this code "as-is" and it looks correct to me:

            [
                'name'       => __( 'Controller', 'your-text-domain' ),
                'id'         => $prefix . 'controller',
                'type'       => 'post',
                'post_type'  => ['controller'],
                'field_type' => 'select',
                'query_args' => [
                    'orderby'     => 'post_title',
                    'order'       => 'ASC',
                    'post_parent' => $post_id,
                ],
                'tab'        => 'monitor_details',

Here's a screenshot of what I'm trying to do: https://pasteboard.co/hquFYGyQQILC.png