I'm using `FBSDKCoreKit` framework for custom login. I followed the documentation perfectly. I'm using `FBSDKLoginManager` for Login.I'm unable to find why this exception was coming and getting exception as 
`[FBSDKInternalUtility JSONStringForObject:error:]:` unrecognized selector sent to class.please help me to solve this issue.

Here is my code:

```
[FBSDKProfile enableUpdatesOnAccessTokenChange:YES];
FBSDKLoginManager *login = [[FBSDKLoginManager alloc] init];
[login logInWithReadPermissions:@[@"email",@"public_profile"] handler:^(FBSDKLoginManagerLoginResult *result, NSError *error) {
    if (error)
    {

    } else if (result.isCancelled)
    {

    } else
    {
        NSLog(@"%@",result);
        if ([result.grantedPermissions containsObject:@"email"])
        {
            if ([FBSDKAccessToken currentAccessToken])
            {
                [[[FBSDKGraphRequest alloc] initWithGraphPath:@"me" parameters:nil]
                 startWithCompletionHandler:^(FBSDKGraphRequestConnection *connection, id result, NSError *error) {
                     if (!error) {

                         NSLog(@"find result=%@",result);
                     }
                 }];
            }

        }
    }
}];

```