Steps to reproduce the issue

• Edit existing menu item
• Change the "Menu Item Type" to "System Links -> Menu Item Alias"
• Click on the "Select" button next to the "Menu Item" property

Expected result

A popup window should appear listing menu items.

Actual result

A popup window appears showing the Administrator interface and an error message:

An error has occurred.
500 The Menu type doesn't exist.

System information (as much as possible)

Joomla 3.8.

Additional comments

I upgraded from 3.7.5 to 3.8 this morning. I don't know whether the update triggered this issue. The last time I modified menu item alias links was in the 3.6 series. This happens both with the old and new style routing enabled.