Error Building Player - iOS

I just updated unity and rebuilt my project into a new Xcode project. I made changes in game but when I try to append the new Xcode project I get 2 errors, one is a NullReferenceException without any log:

 Error building Player: ArgumentNullException: Argument cannot be null.
 Parameter name: key

and the second is:

 ArgumentNullException: Argument cannot be null.
 Parameter name: key
 System.Collections.Generic.Dictionary`2[System.String,UnityEditor.iOS.Xcode.PBXBuildFile].set_Item (System.String key, UnityEditor.iOS.Xcode.PBXBuildFile value) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System.Collections.Generic/Dictionary.cs:155)

I googled and it seems this bug has been around for some time now.

WORKAROUND:

build the project into a new folder, or use Replace instead of Append. Then copy the Data and Library folders from the new Xcode project to the old one. but the bug is still a pain in the neck. if anyone knows how to fix it please help.