The problem is just like it says... you can't insert a NULL value when the table is expecting a number.... presumably it's one of the dropdown you've got there that's expecting a number, only nothing was sleected, so it tried to insert a NULL value instead....only that doesn't work.
You'll need to look to see what events fire off when... see if there's a before move, or something that fires before actually moving to a new row. From there you should be able to see everything going on, and validate your data before moving to the new row.

-tg