Facebook Login Error - Unknown Error building URL (com.facebook.sdk.core error 3)

I found the problem. It's not code related. It seems to be a bug in the latest two versions of Facebook SDK so I downgraded it to

pod 'FBSDKCoreKit', '4.36.0'  
pod 'FBSDKLoginKit', '4.36.0'  
pod 'FacebookCore', '0.4'  
pod 'FacebookLogin', '0.4'

and it run smoothly. Dough for somebody version 4.38 solved the issue it wasn't the car for me. Hope this will be useful to others.

This is a Facebook SDK bug in version 4.39.0 which is causing this error. In order to solve this bug, simply downgrade both CoreKit and LoginKit to 4.38.0, clear derived data as well as clean build folder (CMD + OPTION + SHIFT + K).

pod 'FBSDKCoreKit', '~> 4.38.0'
pod 'FBSDKLoginKit', '~> 4.38.0'

If you are using FacebookCore and FacebookLogin then do like the following.

pod 'FacebookCore'
pod 'FacebookLogin'
pod 'FBSDKCoreKit', '~> 4.38.0'
pod 'FBSDKLoginKit', '~> 4.38.0'

Looks like there has been a hot fix for this 4.39.1 was released Jan 15, 2019 (a week in the future apparently)

https://developers.facebook.com/docs/ios/change-log-4x/