Tested on XWiki 8.4.5, 11.3.4 and 11.7 and the Internal link can be added on a specific text but after Save & View, nothing happens when the link is clicked. It works for external links.
On Cloud (10.11.9) it seems the Internal link can't be added at all - after selecting to link to a specific page from the picker, nothing happens when clicking the Apply button. The external link works.
* Clear the selection and collapse the tree whenever the Edit Link dialog is opened for a wiki link
* Add support for removing the link (by not selecting any node from the tree or by leaving the input empty)