Making NSNull behave like nil. See blog post ronaldmannak.github.com/2013/01/14/nsnull.html

NSNull+behave.m

//
//  NSNull+behave.m
//
//  Created by Ronald Mannak on 1/2/13.
//  Copyright (c) 2013 Ronald Mannak. All rights reserved.
//

@implementation NSNull (catch)

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector {
    
    // If NSNull doesn't respond to aSelector, signature will be nil and a new signature for an empty method
    // will be created and returned
    
    NSMethodSignature *signature = [super methodSignatureForSelector:aSelector];
    
    if (!signature) {
        // Note: "@:" are (id)self and (SEL)_cmd
        signature = [NSMethodSignature signatureWithObjCTypes:"@:"];
    }
    
    return signature;
}

- (void)forwardInvocation:(NSInvocation *)anInvocation
{
    
    // Called if NSNull received a message to a non-existent method
    // Reroute the message to nil
    
    [anInvocation invokeWithTarget:nil];
}

@end