get_current_screen() returns null in ajax requests

get_current_screen().php

/**
 * Let's say you're parsing query args in the admin so 
 * that you can apply some custom filters for your
 * custom post type list table.
 */
add_filter( 'parse_query', 'filter_my_post_type' );
function filter_my_post_type( $query ) {

	/**
	 * Oh good, you've put in that is_admin() check
	 * so it won't effect the front-end.
	 */
	if ( !is_admin() || empty( $query->query['post_type'] ) || $query->query['post_type'] !== 'my_post_type' ) {
		return;
	}

	/**
	 * So we know we're in the admin now, right?
	 * Let's get that screen to make sure we're
	 * only effecting queries on our post type's
	 * list table page.
	 */
	$screen = get_current_screen();
	if ( $screen->post_type == 'my_post_type' ) {
		/**
		 * Woops, someone just queried your
		 * custom post type in an ajax call
		 * from the Customimzer. The
		 * ajax call passes the is_admin()
		 * check but no screen exists. WP_DEBUG
		 * was enabled so you just caused a
		 * PHP Notice that breaks the JSON
		 * response to the ajax call.
		 *
		 * Check you have a screen object
		 * before relying on it!
		 *
		 * is_object( $screen )
		 */
	}
}