ofd文件，cairo_draw时，会出现node为None的情况

[JimmyXC]

报错信息如下：
Traceback (most recent call last):
File "E:\ofd2img-master\ofd_test.py", line 11, in
doc.draw_document()
File "E:\ofd2img-master\core\document.py", line 48, in draw_document
paths.append(surface.draw(page))
File "E:\ofd2img-master\core\document.py", line 199, in draw
self.cairo_draw(self.cr, self.page.tpl_node)
File "E:\ofd2img-master\core\document.py", line 160, in cairo_draw
if node.tag in CAIRO_TAGS:
AttributeError: 'NoneType' object has no attribute 'tag'