[auth] Fix Multi-factor session crash on second Firebase app (#14238)

Co-authored-by: Nick Cooke <36927374+ncooke3@users.noreply.github.com>

Author: paulb777

FirebaseAuth/CHANGELOG.md

@@ -1,4 +1,5 @@
 # Unreleased
+- [fixed] Fix Multi-factor session crash on second Firebase app. (#14238)
 - [fixed] Updated most decoders to be consistent with Firebase 10's behavior
   for decoding `nil` values. (#14212)
 

FirebaseAuth/Sources/Swift/MultiFactor/MultiFactor.swift

@@ -34,7 +34,7 @@ import Foundation
     /// operation.
     @objc(getSessionWithCompletion:)
     open func getSessionWithCompletion(_ completion: ((MultiFactorSession?, Error?) -> Void)?) {
-      let session = MultiFactorSession.sessionForCurrentUser
+      let session = MultiFactorSession.session(for: user)
       if let completion {
         completion(session, nil)
       }

FirebaseAuth/Sources/Swift/MultiFactor/MultiFactorSession.swift

@@ -39,8 +39,9 @@ import Foundation
     /// Current user object.
     var currentUser: User?
 
-    class var sessionForCurrentUser: MultiFactorSession {
-      guard let currentUser = Auth.auth().currentUser else {
+    class func session(for user: User?) -> MultiFactorSession {
+      let currentUser = user ?? Auth.auth().currentUser
+      guard let currentUser else {
         fatalError("Internal Auth Error: missing user for multifactor auth")
       }
       return .init(idToken: currentUser.tokenService.accessToken, currentUser: currentUser)