fix(ui): allow null countryCode for phone input

Author: lesnitsky

packages/firebase_ui_auth/lib/src/views/phone_input_view.dart

@@ -121,7 +121,7 @@ class _PhoneInputViewState extends State<PhoneInputView> {
               widget.subtitleBuilder!(context),
             if (state is AwaitingPhoneNumber || state is SMSCodeRequested) ...[
               PhoneInput(
-                initialCountryCode: countryCode!,
+                initialCountryCode: countryCode,
                 onSubmit: onSubmit(ctrl),
                 key: phoneInputKey,
               ),

packages/firebase_ui_auth/lib/src/widgets/phone_input.dart

@@ -85,7 +85,7 @@ class PhoneInput extends StatefulWidget {
 
   /// An initial country code that should be selected in the country code
   /// picker.
-  final String initialCountryCode;
+  final String? initialCountryCode;
 
   /// Returns a phone number from the [PhoneInput] that was provided a [key].
   static String? getPhoneNumber(GlobalKey<PhoneInputState> key) {
@@ -101,7 +101,7 @@ class PhoneInput extends StatefulWidget {
   /// {@macro ui.auth.widgets.phone_input}
   const PhoneInput({
     Key? key,
-    required this.initialCountryCode,
+    this.initialCountryCode,
     this.onSubmit,
   }) : super(key: key);
 

packages/firebase_ui_auth/test/widgets/phone_input_test.dart

@@ -0,0 +1,34 @@
+import 'package:firebase_ui_auth/src/widgets/phone_input.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  group('PhoneInput', () {
+    testWidgets('shows default country and country code', (tester) async {
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Scaffold(
+            body: PhoneInput(initialCountryCode: 'US'),
+          ),
+        ),
+      );
+
+      expect(find.text('United States'), findsOneWidget);
+    });
+
+    testWidgets(
+      'prompts to select a country if initialCountryCode is null',
+      (tester) async {
+        await tester.pumpWidget(
+          const MaterialApp(
+            home: Scaffold(
+              body: PhoneInput(initialCountryCode: null),
+            ),
+          ),
+        );
+
+        expect(find.text('Choose a country'), findsOneWidget);
+      },
+    );
+  });
+}