[NSNull length]: unrecognized selector sent to instance

[NSNull length]: unrecognized selector sent to instance 백그라운드에서 얻은 데이터가 NSNULL이 있기 때문에 dictionary objectForkey: 작업을 진행했습니다.
해결 방법: 얻은 데이터를 판단할 수 있습니다.null이면 @ "지불 코드로 전환합니다

+(id)changeType:(id)myObj
{
    if ([myObj isKindOfClass:[NSDictionary class]])
    {
        return [self nullDic:myObj];
    }
    else if([myObj isKindOfClass:[NSArray class]])
    {
        return [self nullArr:myObj];
    }
    else if([myObj isKindOfClass:[NSString class]])
    {
        return [self stringToString:myObj];
    }
    else if([myObj isKindOfClass:[NSNull class]])
    {
        return [self nullToString];
    }
    else
    {
        return myObj;
    }
}

// NSDictionary  Null        @""
+(NSDictionary *)nullDic:(NSDictionary *)myDic
{
    NSArray *keyArr = [myDic allKeys];
    NSMutableDictionary *resDic = [[NSMutableDictionary alloc]init];
    for (int i = 0; i < keyArr.count; i ++)
    {
        id obj = [myDic objectForKey:keyArr[i]];
        
        obj = [self changeType:obj];
        
        [resDic setObject:obj forKey:keyArr[i]];
    }
    return resDic;
}

// NSArray  Null        @""
+(NSArray *)nullArr:(NSArray *)myArr
{
    NSMutableArray *resArr = [[NSMutableArray alloc] init];
    for (int i = 0; i < myArr.count; i ++)
    {
        id obj = myArr[i];
        
        obj = [self changeType:obj];
        
        [resArr addObject:obj];
    }
    return resArr;
}
// NSString       
+(NSString *)stringToString:(NSString *)string
{
    return string;
}

// Null        @""
+(NSString *)nullToString
{
    return @"";
}