Steps to reproduce the issue

It is not allowed that a menu item alias is the same as a sub-folder in your Joomla installation.
The message will be: "Save failed with the following error: A first level menu item alias can't be 'tmp' because 'tmp' is a sub-folder of your joomla installation folder."
My problem is that when you enter a new menu item this check is not done, before creating the menu item.
So you can create a menu item with the name of a sub-folder. The moment the item is creacted you can't save it again.

Expected result

Check before the item is created.

Actual result

No check and an error on the frontend of the site because your not allowed to browse a sub-folder.

System information (as much as possible)

Additional comments