Notice: Undefined variable $link com menus
From Joomla! Documentation
In Joomla 3.4.2 we improved the usage of modals also in menus.
This pertains only to Joomla! version(s): 3.4.3
What is the cause
You get a notice like:
Notice: Undefined variable: link in JROOT/administrator/components/com_menus/views/menus/tmpl/default.php on line 207
But adding a module for the menu should still work.
How to fix
Apply the patch https://github.com/joomla/joomla-cms/pull/7344.diff.
This will be included in Joomla 3.4.4