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)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)aKey
{
	if(anObject!=nil) {
		[self setObject:anObject forKey:aKey];
		return YES;
	}
	else {
		[self setObject:[NSNull null] forKey:aKey];
		return NO;
	}
}

@end