It appears there is a bug in the Designer where assigning a jsClass name where the first character is lowercase results in an error. This is because the var reference name that gets used in one of the .js files then happens to be the name of the class - name collision!
I'd file a bug report on this. However, best practice oo programming is usually to begin a class name with an uppercase character, anyway.
Change your jsClass on the ViewPort to have an uppercase first character - like MyViewPort or something - and you should be fine.