PHP Notice Breaks Admin AJAX

When WP_DEBUG is set to true the plugin throws PHP notice “Notice: ob_end_flush(): failed to delete and flush buffer. No buffer to delete or flush in /wp-content/plugins/html-cleanup/html-cleanup.php on line 45” which breaks all admin AJAX functionality. Disabling the plugin or changing WP_DEBUG to false makes it work again. The problem code:

function html_cleanup_buffer_end() {
  ob_end_flush();
}

Wrapping the function with output buffer check fixes this:

function html_cleanup_buffer_end() {
  if(ob_get_level() !== 0) {
    ob_end_flush();
  }
}

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