[notebook_shortcode]object has no attribute 'handler'

[Further]

Traceback (most recent call last):
File "/home/user/lib/bin/nikola", line 8, in
sys.exit(main())
File "/home/user/lib/lib/python3.8/site-packages/nikola/main.py", line 166, in main
_ = DN.run(oargs)
File "/home/user/lib/lib/python3.8/site-packages/nikola/main.py", line 352, in run
self.nikola.init_plugins(load_all=True)
File "/home/user/lib/lib/python3.8/site-packages/nikola/nikola.py", line 1176, in init_plugins
self._activate_plugins_of_category("ShortcodePlugin")
File "/home/user/lib/lib/python3.8/site-packages/nikola/nikola.py", line 1334, in _activate_plugins_of_category
plugin_info.plugin_object.set_site(self)
File "/home/user/docs/plugins/notebook_shortcode/notebook_shortcode.py", line 47, in set_site
super(NotebookShortcodePlugin, self).set_site(site)
File "/home/user/lib/lib/python3.8/site-packages/nikola/plugin_categories.py", line 492, in set_site
site.register_shortcode(self.name, self.handler)
AttributeError: 'NotebookShortcodePlugin' object has no attribute 'handler'