Q:

How to check if NSDictionary key is NULL

I'm trying to check if an object exist for the key next_max_id in my JSON response dictionary. 
I've tried the following ways to check:

if ([[pagination objectForKey:@"next_max_id"] isKindOfClass:[NSNull class]]) ...
if ([pagination objectForKey:@"next_max_id"] == [NSNull class]) ...
if ([pagination objectForKey:@"next_max_id"]) ...

However, when the pagination dictionary is empty i.e. (pagination = {};), I get these error messages:
-[NSNull objectForKey:]: unrecognized selector sent to instance ...
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull objectForKey:]: unrecognized selector sent to instance ...

How else can I check for if the object or rather the key exist, without a crash? 
Updated:
When the key next_max_id exist inside pagination, the JSON response would be like this 
"pagination": {
    "next_url": "https://api.instagram.com/v1/tags/puppy/media/recent?access_token=fb2e77d.47a0479900504cb3ab4a1f626d174d2d&max_id=13872296",
    "next_max_id": "13872296"
}

A:

if( [pagination objectForKey:@"your_key"] == nil ||  
    [[pagination objectForKey:@"your_key"] isEqual:[NSNull null]] ){
    //nil dictionary
}

A:

With the error that you describe, pagination is not an NSDictionary, but pagination itself is an NSNull object.  So the first check before everything else would be
if (pagination == [NSNull null]) ...

That wouldn't happen if your JSON data contains "pagination": {} but it would happen if your JSON data contains "pagination": null . 