Summary:
Changelog: [internal]
[NSString UTF8String] is nullable. Therefore, we need to check if it isn't nil before passing it to std::string constructor which crashes if it's nil.
Reviewed By: shergin
Differential Revision: D23572652
fbshipit-source-id: 59e7f5e918b2e5c69333bfb687371f856555d8e0