Fix RadioGroup single selection check. (flutter#175654)

Fixes flutter#175258

### Description

This PR fixes the single selection check in `RadioGroup` by delaying it
to the next frame, which ensures that all `registerClient` and
`unregisterClient` calls are processed.

The same pattern can be found in
`_TabBarState._debugScheduleCheckHasValidTabsCount` and
`RawScrollbarState._debugScheduleCheckHasValidScrollPosition`.

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [X] All existing and new tests are passing.


<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: ksokolovskyi

packages/flutter/lib/src/widgets/radio_group.dart

@@ -9,6 +9,7 @@ import 'package:flutter/services.dart';
 
 import 'actions.dart';
 import 'basic.dart';
+import 'binding.dart';
 import 'focus_manager.dart';
 import 'focus_traversal.dart';
 import 'framework.dart';
@@ -96,6 +97,27 @@ class _RadioGroupState<T> extends State<RadioGroup<T>> implements RadioGroupRegi
 
   final Set<RadioClient<T>> _radios = <RadioClient<T>>{};
 
+  bool _debugHasScheduledSingleSelectionCheck = false;
+
+  /// Schedules a check for the next frame to verify that there is only one
+  /// radio with the group value.
+  bool _debugScheduleSingleSelectionCheck() {
+    if (_debugHasScheduledSingleSelectionCheck) {
+      return true;
+    }
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      _debugHasScheduledSingleSelectionCheck = false;
+      if (!mounted || _debugCheckOnlySingleSelection()) {
+        return;
+      }
+      throw FlutterError(
+        "RadioGroupPolicy can't be used for a radio group that allows multiple selection.",
+      );
+    }, debugLabel: 'RadioGroup.singleSelectionCheck');
+    _debugHasScheduledSingleSelectionCheck = true;
+    return true;
+  }
+
   bool _debugCheckOnlySingleSelection() {
     return _radios.where((RadioClient<T> radio) => radio.radioValue == groupValue).length < 2;
   }
@@ -106,10 +128,7 @@ class _RadioGroupState<T> extends State<RadioGroup<T>> implements RadioGroupRegi
   @override
   void registerClient(RadioClient<T> radio) {
     _radios.add(radio);
-    assert(
-      _debugCheckOnlySingleSelection(),
-      "RadioGroupPolicy can't be used for a radio group that allows multiple selection",
-    );
+    assert(_debugScheduleSingleSelectionCheck());
   }
 
   @override
@@ -176,6 +195,8 @@ class _RadioGroupState<T> extends State<RadioGroup<T>> implements RadioGroupRegi
 
   @override
   Widget build(BuildContext context) {
+    assert(_debugScheduleSingleSelectionCheck());
+
     return Semantics(
       container: true,
       role: SemanticsRole.radioGroup,

packages/flutter/test/widgets/radio_group_test.dart

@@ -244,6 +244,89 @@ void main() {
     expect(radio1.hasFocus, isTrue);
     expect(textFieldAfter.hasFocus, isFalse);
   });
+
+  // Regression test for https://github.com/flutter/flutter/issues/175258.
+  testWidgets('Radio group throws on multiple selection', (WidgetTester tester) async {
+    final UniqueKey key1 = UniqueKey();
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: TestRadioGroup<int>(
+            child: Column(
+              children: <Widget>[
+                const Radio<int>(value: 0),
+                Radio<int>(key: key1, value: 1),
+                const Radio<int>(value: 1),
+                const Radio<int>(value: 2),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isNull);
+
+    await tester.tap(find.byKey(key1));
+    await tester.pump();
+
+    expect(
+      tester.takeException(),
+      isA<FlutterError>().having(
+        (FlutterError e) => e.message,
+        'message',
+        "RadioGroupPolicy can't be used for a radio group that allows multiple selection.",
+      ),
+    );
+  });
+
+  // Regression test for https://github.com/flutter/flutter/issues/175258.
+  testWidgets('Radio group does not throw when number of children decreases', (
+    WidgetTester tester,
+  ) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: RadioGroup<int>(
+            onChanged: (_) {},
+            groupValue: 4,
+            child: const Column(
+              children: <Widget>[
+                Radio<int>(value: 0),
+                Radio<int>(value: 1),
+                Radio<int>(value: 2),
+                Radio<int>(value: 3),
+                Radio<int>(value: 4),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isNull);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Material(
+          child: RadioGroup<int>(
+            onChanged: (_) {},
+            groupValue: 4,
+            child: const Column(
+              children: <Widget>[
+                Radio<int>(value: 1),
+                Radio<int>(value: 2),
+                Radio<int>(value: 3),
+                Radio<int>(value: 4),
+              ],
+            ),
+          ),
+        ),
+      ),
+    );
+
+    expect(tester.takeException(), isNull);
+  });
 }
 
 class TestRadioGroup<T> extends StatefulWidget {