-[__NSArrayM insertObject:atIndex:]: object cannot be nil

-[__NSArrayM insertObject:atIndex:]: object cannot be nil

I am getting error object cannot be nil. I am using web service, when I
parse it using loop it crashed and send output -[__NSArrayM
insertObject:atIndex:]: object cannot be nil as error, because I have null
value on blank value in NSMutableArray which I am trying to parse. I tried
to fix with (1) if([Array containsObject:[NSNull null]]) (2) if([Array
objectAtIndex:0 (3) if([Array objectAtIndex:counter] == 0. methods.
My code is,
if([[[node childAtIndex:counter] name] isEqualToString:@"Phone"]){
if([phoneno containsObject:[NSNull null]]){
phone.text = @"-";
}
else{
NSString *str = [[node childAtIndex:counter]
stringValue];
[phoneno addObject:str];
NSString *myString = [phoneno
componentsJoinedByString:@","];
[phone setText:myString];
phone.text = [NSString stringWithFormat:@"%@",myString];
}
}
Here, phoneno is NSMutableArray
What am I doing wrong?