iOS [NSKeyedUnarchiver initForReadingWithData:]: data is NULL?

解决方法：

1、if ([[[NSUserDefaults standardUserDefaults] dictionaryRepresentation].allKeys containsObject:@"keyForNonMandatoryObject"]) {

        // unarchive the value here

 }

2、NSUserDefaults * defaults = [NSUserDefaults standardUserDefaults];

[defaults setObject:[NSKeyedArchiver archivedDataWithRootObject:myObject] forKey:@"MyObjectKey"]; 

[defaults synchronize];