Errors when you use bulk action to add-to-group but don't select a group

[tbar0970]

Steps to reproduce:
Go to persons --> list all
Select all
At the bottom choose "with selected persons... add to group"
DOn't select any group
Click Go.
PHP errors ensue.

It should make sure you've selected a group, or at least handle it better on the server side with an
add_message("You did not select a group", 'error');