View file upload/library/XenForo/ViewPublic/Register/Form.php

<?php

class XenForo_ViewPublic_Register_Form extends XenForo_ViewPublic_Base
{
	public function renderHtml()
	{
		if (!empty($this->_params['captcha']))
		{
			$this->_params['captcha'] = $this->_params['captcha']->render($this);
		}
	}
}