first portion of If statement is being passed over; optional error

First off, here is a link to my simple little GPA project: http://ift.tt/1UrSnNv . I understand that if you do not wish to download,i'll include pictures as well.

Anyways, I have a simple 4 text field, 1 button, and 1 label set up. I thought I had my code set up successfully that if each text field was not holding a value, a ui alert value would pop up asking for a value in each text box. If there are 4 values, then the app would complete the "math." However, when I leave all 4 empty or if I click the button when all 4 are filled and then delete 1 text field, my app crashes... "unexpectedly found nil while unwrapping an Optional value". I find it frustrating (which could be a simple mistake on my end, but in my mind I shouldn't be having this value this my pop up alert is meant to be used when a text field is nil.

Thank you for your time, Jon.