Xcode 8 - Found nil while unwrapping an Optional value - cannot find the crashing line

fatal error: unexpectedly found nil while unwrapping an Optional value

I cannot see the crashing line because of the last "message from debugger":

Message from debugger: The LLDB RPC server has crashed. Please file a bug with Apple with the crash log.

It just crashes and I can't debug my app. I've tried putting some "debug prints" in the code but it is just impossible to find the bug.

BTW, I use the same code as I used before Swift 3, and this nil error wasn't there.

If it helps, I use FBSDK and Alamofire.