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

Question

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.

As the error states, there was some issue in the debugger. You need to file a bug to apple. Remember Xcode 8 is still under beta and can have several issues. So don't trust it 100% for the time being. :) — Midhun MP
I wish I could use Xcode 7, but the simulator doesn't work on macOS Sierra... Thanks! — Etgar

Michael Lee Michael Lee · Accepted Answer · 2016-06-19T07:23:16

I encountered this error, and it was resolved after I followed the steps in the link below:

(I was trying to solve a separate issue related to the simulator)

I'm not sure if the two issues are related, but it might help you.