I believe the message is a warning (yellow triangle with "!"). A real error will halt execution (when doing Ctrl+F5 run w/no debug).
How to handle? A couple of options.
1. Leave it alone for now. There are enough issues you're going to deal with in NerdDinner.
2. Relax the constraint
3. Turn off the warning. It's an option under Tools / Options / Debugging.
4. Handle the exception. You'll find there not much you can do with this (in this case) other than have an empty handler to make the debugger happy. In addition there will be lot's of other unhandled exceptions. Do you really want to deal with them all right now?
5. Make the column nullable in the database. Redo the Dinner entity. Then Let MVC do the validations. Works, but not a good idea.
Hope this helps.