It looks like your script is mostly right, but are you trying to do something extra in the message box? Did you mean to have the dropdown name in the title or as part of the message?
If you aren't allowing custom entry, you only need the null check. Of course, we're only talking about a millisecond or so to check the length of your string. So, you aren't asking any devices to do intense processing.
If I were asked to do this, this is what my script would look like.


If you are allowing custom entries, then you should specify what the problem is for the user.

