400 Bad Request, in wordpress theme development, wp_ajax

The problem is that, you have added extra white space before wp_ajax action. remove space from actions ' wp_ajax_my_action' and ' wp_ajax_nopriv_my_action'. because of extra space before ajax request, showing the 400 bad request error.
You should use :

add_action('wp_ajax_my_action', 'my_action');
add_action('wp_ajax_nopriv_my_action', 'my_action');