将NSDictionary存储在永久内存中NSUserDefaults会抛出setValue：forUndefinedKey

I'm trying to optimize my app since the loading time seems to be very slow. Right now I'm working on a newsfeed. The first thing I'm doing is saving a Dictionary of [followingId : followingPFUser] with the id and the PFUser that corresponds to that userId. For some reasons it gives me this error when I run the function : I'm trying to optimize my app since the loading