This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I actually see only one form on there - as delineated by <form> </form> tags. So you don't have multiple forms. If you want multiple forms, you will need to put each one in their own set of form tags and give each one their own submit button.
The error tells you that you are trying to pull out a value for a field and turn it into a number, but the field was not entered by the user. So you have to validate the form (at least check that all required fields were entered) and respond to the client that there are missing values if they haven't filled the form out completely.