Properly throw error when CupertinoPageRoute builder returns null (#42602)

packages/flutter/lib/src/cupertino/route.dart

@@ -253,17 +253,18 @@ class CupertinoPageRoute<T> extends PageRoute<T> {
 
   @override
   Widget buildPage(BuildContext context, Animation<double> animation, Animation<double> secondaryAnimation) {
+    final Widget child = builder(context);
     final Widget result = Semantics(
       scopesRoute: true,
       explicitChildNodes: true,
-      child: builder(context),
+      child: child,
     );
     assert(() {
-      if (result == null) {
-        throw FlutterError(
-          'The builder for route "${settings.name}" returned null.\n'
-          'Route builders must never return null.'
-        );
+      if (child == null) {
+        throw FlutterError.fromParts(<DiagnosticsNode>[
+          ErrorSummary('The builder for route "${settings.name}" returned null.'),
+          ErrorDescription('Route builders must never return null.'),
+        ]);
       }
       return true;
     }());

packages/flutter/test/cupertino/route_test.dart

@@ -15,6 +15,34 @@ void main() {
     navigatorObserver = MockNavigatorObserver();
   });
 
+  testWidgets(
+    'Throws FlutterError with correct message when route builder returns null',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const CupertinoApp(
+        home: Placeholder(),
+      ),
+    );
+
+    tester.state<NavigatorState>(find.byType(Navigator)).push(
+          CupertinoPageRoute<void>(
+            title: 'Route 1',
+            builder: (_) => null,
+          ),
+        );
+
+    await tester.pump();
+    await tester.pump(const Duration(milliseconds: 500));
+
+    final dynamic error = tester.takeException();
+    expect(error, isFlutterError);
+    expect(error.toStringDeep(), equalsIgnoringHashCodes(
+      'FlutterError\n'
+      '   The builder for route "null" returned null.\n'
+      '   Route builders must never return null.\n'
+    ));
+  });
+
   testWidgets('Middle auto-populates with title', (WidgetTester tester) async {
     await tester.pumpWidget(
       const CupertinoApp(