[REQUIRED] Step 2: Describe your environment

• Xcode version: 11.1 (11A1027)
• Firebase SDK version: 6.11
• Firebase Component: Auth
• Component version: 6.3.1

[REQUIRED] Step 3: Describe the problem

Steps to reproduce:

1. Follow the steps in Enabling cross-app authentication with shared iOS Keychain
2. Do the following:

// ... inside application(_:didFinishLaunchingWithOptions:):
FirebaseApp.configure()

do {
    try Auth.auth().useUserAccessGroup("TEAMID.com.example.group1")
    print("useUserAccessGroup OK")
} catch let error as NSError {
    print("useUserAccessGroup failed with \(error)")
}

Expected output:
useUserAccessGroup OK

Actual output:
useUserAccessGroup failed with Error Domain=Foundation._GenericObjCError Code=0 "(null)"

My analysis

It seems like there is some async setup going on which prevents this from working. If I wrap the useUserAccessGroup call with

DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
	do {
	    try Auth.auth().useUserAccessGroup("TEAMID.com.example.group1")
	    print("useUserAccessGroup OK")
	} catch let error as NSError {
	    print("useUserAccessGroup failed with \()")
	}
}

or move it out of application(_:didFinishLaunchingWithOptions:): and call it later it works as expected.

I think this is a bug, but if it's not, I would suggest improving the error message to something more helpful.