NSMutableDictionary category to handle setting nil as an object for a key, adding [NSNull null] instead.

NSMutableDictionary+PXDictionaryAdditions.h

#import <Cocoa/Cocoa.h>

@interface NSMutableDictionary (PXDictionaryAdditions)
    - (BOOL)setObjectOrNull:(id)anObject forKey:(id <NSCopying>)aKey;
@end

NSMutableDictionary+PXDictionaryAdditions.m

#import "NSMutableDictionary+PXDictionaryAdditions.h"

@implementation NSMutableDictionary (PXDictionaryAdditions)

//Returns NO if `anObject` is nil; can be used by the sender of the message or ignored if it is irrelevant.
- (BOOL)setObjectOrNull:(id)anObject forKey:(id <NSCopying>)aKey {
    if(!anObject) {
        [self setObject:[NSNull null] forKey:aKey];
        return NO;
    }
    [self setObject:anObject forKey:aKey];
    return YES;
}

@end