Follow this senario, please, for better understanding:
Go to admin.php and create new user page. Finish it however you like and place it anywhere on the Main Menu.
Notice that it works fine.
Go to admin.php and Edit User Page link; so as to edit this page.
Delete the page.
Go to Main Menu and see that it is still there. When selected you find an empty page with "Edit this page..." on top.
There needs to be logic that takes this out of Main Menu.