private function unregister_uninstall_hook() {
            $uninstallable_plugins = (array) get_option( 'uninstall_plugins' );
            unset( $uninstallable_plugins[ $this->_free_plugin_basename ] );
            unset( $uninstallable_plugins[ $this->premium_plugin_basename() ] );

            update_option( 'uninstall_plugins', $uninstallable_plugins );
        }