NSSet to separate strings by comma

I have the next code for converting NSSet to string separating by comma:

-(NSString *)toStringSeparatingByComma
    NSMutableString *resultString = [NSMutableString new];
    NSEnumerator *enumerator = [self objectEnumerator];
    NSString* value;
    while ((value = [enumerator nextObject])) {
        [resultString appendFormat:[NSString stringWithFormat:@" %@ ,",value]];//1

    NSRange lastComma = [resultString rangeOfString:@"," options:NSBackwardsSearch];
    if(lastComma.location != NSNotFound) {
        resultString = [resultString stringByReplacingCharactersInRange:lastComma  //2
                                                             withString: @""];
    return resultString;

It seems that it works, but I get here two warnings:

1. format string is not a string literal (potentially insecure)

2. incompatible pointer types assigning to nsmutablestring from nsstring

How to rewrite it to avoid of warnings?

There is another way to achieve what you are trying to do with fewer lines of code:

You can get an array of NSSet objects using:

NSArray *myArray = [mySet allObjects];

You can convert the array to a string:

NSString *myStr = [myArray componentsJoinedByString:@","];