Testing for nothing causing an error as if it were nothing

I have a routine that essentially assigned a class to a session property.
It is supposed to detect when no class has yet been created for this session and then do so.
It usually works fine but in certain situations that I can't recoginse it returns nothing and yet I cannot seem to test for that eventuality.
I could use a error catch but I would like to understand why testing if something is nothing can cause this error.  As you can see I test if the Current object is also nothing before accessing the session object.
I would appreciate any insights on how this can occur when I am literally trying to detect the error it raises.  
Thanks,