iOS - App Crashes with compelationHandler:nil - ios

Question

Hey so I am getting the classic EXC_BAD_ACCESS (Code = 1, address = 0x10) and can't seem to figure out why. I get the error with the following method:
+ (void)logoutWithXId:(NSString *)xId compelationHandler:(void (^)(BOOL))hasSucceeded
{
NSError *error;
// create json object for a users session
NSDictionary *session = [NSDictionary dictionaryWithObjectsAndKeys:
xId, #"x_id",
nil];
NSData *jsonSession = [NSJSONSerialization dataWithJSONObject:session options:NSJSONWritingPrettyPrinted error:&error];
NSString *url = [NSString stringWithFormat:#"%#xsession/logout", potCatURL];
NSURL *URL = [NSURL URLWithString:url];
NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:URL
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:30.0];
NSString *headerValue = [NSString stringWithFormat:#"Token token=%#", APIToken];
[request setHTTPMethod:#"POST"];
[request setValue:#"application/json" forHTTPHeaderField:#"Accept"];
[request setValue:#"application/json" forHTTPHeaderField:#"Content-Type"];
[request setValue:headerValue forHTTPHeaderField:#"Authorization"];
[request setValue:[NSString stringWithFormat:#"%lu", (unsigned long)[jsonSession length]] forHTTPHeaderField:#"Content-Length"];
[request setHTTPBody:jsonSession];
[NSURLConnection sendAsynchronousRequest:request queue:[NSOperationQueue mainQueue] completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {
if (error == nil)
{
NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
NSInteger statusCode = httpResponse.statusCode;
if (statusCode == 200)
{
//********************* error happens here *******************************
hasSucceeded(true);
}
else
{
hasSucceeded(false);
}
}
else
{
hasSucceeded(false);
}
}];
}
When I call the method like so:
[Xsession logoutWithXId:[userInfo stringForKey:#"x_id"] compelationHandler:nil]
I get the EXC_BAD_ACCESS error but when I call it like:
[Xsession logoutWithXId:[userInfo stringForKey:#"x_id"] compelationHandler:^(BOOL hasCreated){}]
with an empty compilationHandler it runs fine. Why is this happening, can someone please explain?

Answer 1

It's due to the nil handler. You must always do the following:
if (hasSucceeded) {
hasSucceeded(someValue);
}
The crash is caused by dereferencing the nil block pointer. This is different from trying to call a method on a nil variable reference (which is fine). You must never dereference a nil block pointer.
There is some great information about this in another answer here.

Answer 2

The reason you get a crash is that you don't check whether passed argument is nil or not. Blocks in this case are different (you should treat it like a pointer to a function not like NSObject subclass). You cannot call a function on address 0.
To fix it you should check if the block is not nil before calling it:
if (hasSucceded) {
hasSucceeded(value);
}