Dropdown add modals not working, JS error

Similar to #2132 , putting any add modal in the dropdown does nothing in the UI and generates the following JS error in the console:

`Uncaught TypeError: can't access property "open", $[PLUGIN_NAME].lookup[$target.data(...)] is undefined`

However, note also:

* with `show_in: bar`, the problem never happens (but the button isn't visible in all contexts, so not a great workaround)
* the error only happens in the context of editing pages, not in the pages navigator
* it's _any_ add modal, for adding modules _or_ pages (unlike #2132).

Firefox browser. Latest Admin and Grav core. Cheers.

Reproduce the error in Firefox while editing pages, using an add modal in a dropdown, and compare it with the working `show_in: bar` case and the pages navigator. Trace the undefined `lookup[$target.data(...)]` entry from the dropdown modal JavaScript; done means module and page add modals open without the console error.

Written by the indexing model from the issue text.