如何写可为空的完成块？

当我用nil调用此方法时，应用程序崩溃，但是我想知道如何用nullable编写它。

崩溃

 [KPTaxnoteApiSaveHandler saveEntryWithUuid:uuid completion:nil];

好

 [KPTaxnoteApiSaveHandler saveEntryWithUuid:uuid completion:^(NSError *error) {}];

这是代码。

+ (void)saveEntryWithUuid:(NSString *)uuid completion:(void (^ __nullable)(NSError * _Nullable error))completion {

    NSLog(@"saveEntryWithUuid");

    Entry *entry = [Entry MR_findFirstByAttribute:@"uuid" withValue:uuid];

    NSDictionary *params = @{@"entry[uuid]":entry.uuid};

    [KPTaxnoteApiSaveHandler postWithUrl:kApiUrlStringForEntry params:params completion:^(NSError *error) {

        if (!error) {

            [MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {

                Entry *entry    = [Entry MR_findFirstByAttribute:@"uuid" withValue:uuid inContext:localContext];
                entry.needSave  = @NO;
            }];
        }

        completion(error);
    }];

+ (void)postWithUrl:(NSString *)urlStr params:(NSDictionary *)params completion:(nullable void (^)(NSError *_Nullable error))completion {

    AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];

    [manager POST:urlStr parameters:params success:^(AFHTTPRequestOperation *operation, id responseObject) {

        completion(nil);

    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {

        completion(error);
    }];