fix(auth): Fix an issue that prevents signInWithWebUI to present over a presenting vc (#635)

Author: royjit

AmplifyPlugins/Auth/AWSCognitoAuthPlugin/Dependency/AuthenticationProviderAdapter+SignIn.swift

@@ -137,7 +137,14 @@ extension AuthenticationProviderAdapter {
         let navController = UINavigationController(rootViewController: UIViewController())
         navController.isNavigationBarHidden = true
         navController.modalPresentationStyle = .overCurrentContext
-        window.rootViewController?.present(navController, animated: false, completion: {
+
+        // Get top most view controller to present a navController
+        var parentViewController = window.rootViewController
+        while ((parentViewController?.presentedViewController) != nil) {
+            parentViewController = parentViewController?.presentedViewController
+        }
+
+        parentViewController?.present(navController, animated: false, completion: {
 
             self.awsMobileClient.showSignIn(navigationController: navController,
                                             signInUIOptions: SignInUIOptions(),