fix(ui_oauth): initialize auth and auth listener in standalone OAuth buttons

packages/firebase_ui_auth/example/test_driver/google_sign_in_test.dart

@@ -88,6 +88,20 @@ void main() async {
         expect(user.displayName, 'Test User');
         expect(user.email, '[email protected]');
       });
+
+      testWidgets('works standalone', (tester) async {
+        await render(
+          tester,
+          const GoogleSignInButton(
+            loadingIndicator: CircularProgressIndicator(),
+            clientId: 'test',
+          ),
+        );
+
+        final button = find.byType(GoogleSignInButton);
+        await tester.tap(button);
+        await tester.pump();
+      });
     },
     skip: !provider.supportsPlatform(defaultTargetPlatform),
   );

packages/firebase_ui_oauth/lib/src/oauth_provider_button_base.dart

@@ -142,6 +142,14 @@ class _OAuthProviderButtonBaseState extends State<OAuthProviderButtonBase>
   double get _height => widget.size + widget._padding * 2;
   late bool isLoading = widget.isLoading;
 
+  @override
+  void initState() {
+    super.initState();
+
+    widget.provider.auth = widget.auth ?? FirebaseAuth.instance;
+    widget.provider.authListener = this;
+  }
+
   void _signIn() {
     final platform = Theme.of(context).platform;
 

packages/firebase_ui_oauth/test/flutterfire_ui_oauth_test.dart

@@ -1,3 +1,4 @@
+import 'package:firebase_auth/firebase_auth.dart' show FirebaseAuth;
 import 'package:flutter/material.dart';
 import 'package:flutter_svg/flutter_svg.dart';
 import 'package:flutter_test/flutter_test.dart';
@@ -43,6 +44,8 @@ class FakeOAuthProvider extends OAuthProvider {
   }
 }
 
+class FakeAuth extends Fake implements FirebaseAuth {}
+
 void main() {
   final provider = FakeOAuthProvider();
 
@@ -54,6 +57,7 @@ void main() {
       provider: provider,
       label: 'Sign in with Google',
       loadingIndicator: const CircularProgressIndicator(),
+      auth: FakeAuth(),
     );
 
     return DefaultAssetBundle(