improve error message for navigator page api (#73153)

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

@@ -3310,10 +3310,35 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
   @override
   void initState() {
     super.initState();
-    assert(
-      widget.pages.isEmpty || widget.onPopPage != null,
-      'The Navigator.onPopPage must be provided to use the Navigator.pages API',
-    );
+    assert((){
+      if (widget.pages != const <Page<dynamic>>[]) {
+        // This navigator uses page API.
+        if (widget.pages.isEmpty) {
+          FlutterError.reportError(
+            FlutterErrorDetails(
+              exception: FlutterError(
+                'The Navigator.pages must not be empty to use the '
+                'Navigator.pages API'
+              ),
+              library: 'widget library',
+              stack: StackTrace.current,
+            ),
+          );
+        } else if (widget.onPopPage == null) {
+          FlutterError.reportError(
+            FlutterErrorDetails(
+              exception: FlutterError(
+                'The Navigator.onPopPage must be provided to use the '
+                'Navigator.pages API'
+              ),
+              library: 'widget library',
+              stack: StackTrace.current,
+            ),
+          );
+        }
+      }
+      return true;
+    }());
     for (final NavigatorObserver observer in widget.observers) {
       assert(observer.navigator == null);
       observer._navigator = this;
@@ -3480,10 +3505,35 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
   @override
   void didUpdateWidget(Navigator oldWidget) {
     super.didUpdateWidget(oldWidget);
-    assert(
-      widget.pages.isEmpty || widget.onPopPage != null,
-      'The Navigator.onPopPage must be provided to use the Navigator.pages API',
-    );
+    assert((){
+      if (widget.pages != const <Page<dynamic>>[]) {
+        // This navigator uses page API.
+        if (widget.pages.isEmpty) {
+          FlutterError.reportError(
+            FlutterErrorDetails(
+              exception: FlutterError(
+                'The Navigator.pages must not be empty to use the '
+                'Navigator.pages API'
+              ),
+              library: 'widget library',
+              stack: StackTrace.current,
+            ),
+          );
+        } else if (widget.onPopPage == null) {
+          FlutterError.reportError(
+            FlutterErrorDetails(
+              exception: FlutterError(
+                'The Navigator.onPopPage must be provided to use the '
+                'Navigator.pages API'
+              ),
+              library: 'widget library',
+              stack: StackTrace.current,
+            ),
+          );
+        }
+      }
+      return true;
+    }());
     if (oldWidget.observers != widget.observers) {
       for (final NavigatorObserver observer in oldWidget.observers)
         observer._navigator = null;
@@ -3494,10 +3544,21 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
       _updateEffectiveObservers();
     }
     if (oldWidget.pages != widget.pages && !restorePending) {
-      assert(
-        widget.pages.isNotEmpty,
-        'To use the Navigator.pages, there must be at least one page in the list.'
-      );
+      assert((){
+        if (widget.pages.isEmpty) {
+          FlutterError.reportError(
+            FlutterErrorDetails(
+              exception: FlutterError(
+                'The Navigator.pages must not be empty to use the '
+                'Navigator.pages API'
+              ),
+              library: 'widget library',
+              stack: StackTrace.current,
+            ),
+          );
+        }
+        return true;
+      }());
       _updatePages();
     }
 

packages/flutter/test/widgets/navigator_test.dart

@@ -2608,6 +2608,79 @@ void main() {
       expect(find.text('initial'), findsOneWidget);
     });
 
+    testWidgets('throw if onPopPage callback is not provided', (WidgetTester tester) async {
+      final List<TestPage> myPages = <TestPage>[
+        const TestPage(key: ValueKey<String>('1'), name:'initial'),
+        const TestPage(key: ValueKey<String>('2'), name:'second'),
+        const TestPage(key: ValueKey<String>('3'), name:'third'),
+      ];
+
+      await tester.pumpWidget(
+        MediaQuery(
+          data: MediaQueryData.fromWindow(WidgetsBinding.instance!.window),
+          child: Localizations(
+            locale: const Locale('en', 'US'),
+            delegates: const <LocalizationsDelegate<dynamic>>[
+              DefaultMaterialLocalizations.delegate,
+              DefaultWidgetsLocalizations.delegate
+            ],
+            child: Directionality(
+              textDirection: TextDirection.ltr,
+              child: Navigator(
+                pages: myPages,
+              ),
+            ),
+          ),
+        )
+      );
+
+      final dynamic exception = tester.takeException();
+      expect(exception, isFlutterError);
+      final FlutterError error = exception as FlutterError;
+      expect(
+        error.toStringDeep(),
+        equalsIgnoringHashCodes(
+          'FlutterError\n'
+          '   The Navigator.onPopPage must be provided to use the\n'
+          '   Navigator.pages API\n'
+          ''
+        ),
+      );
+    });
+
+    testWidgets('throw if page list is empty', (WidgetTester tester) async {
+      final List<TestPage> myPages = <TestPage>[];
+      final FlutterExceptionHandler? originalOnError = FlutterError.onError;
+      FlutterErrorDetails? firstError;
+      FlutterError.onError = (FlutterErrorDetails? detail) {
+        // We only care about the first error;
+        firstError ??= detail;
+      };
+      await tester.pumpWidget(
+          MediaQuery(
+            data: MediaQueryData.fromWindow(WidgetsBinding.instance!.window),
+            child: Localizations(
+              locale: const Locale('en', 'US'),
+              delegates: const <LocalizationsDelegate<dynamic>>[
+                DefaultMaterialLocalizations.delegate,
+                DefaultWidgetsLocalizations.delegate
+              ],
+              child: Directionality(
+                textDirection: TextDirection.ltr,
+                child: Navigator(
+                  pages: myPages,
+                ),
+              ),
+            ),
+          )
+      );
+      FlutterError.onError = originalOnError;
+      expect(
+        firstError!.exception.toString(),
+        'The Navigator.pages must not be empty to use the Navigator.pages API',
+      );
+    });
+
     testWidgets('can push and pop pages using page api', (WidgetTester tester) async {
       late Animation<double> secondaryAnimationOfRouteOne;
       late Animation<double> primaryAnimationOfRouteOne;