Do not animate `TabBarView` if controller is invalid (#123442)

packages/flutter/lib/src/material/tabs.dart

@@ -1893,6 +1893,10 @@ class _TabBarViewState extends State<TabBarView> {
       return false;
     }
 
+    if (!_controllerIsValid) {
+      return false;
+    }
+
     _scrollUnderwayCount += 1;
     if (notification is ScrollUpdateNotification && !_controller!.indexIsChanging) {
       final bool pageChanged = (_pageController.page! - _controller!.index).abs() > 1.0;

packages/flutter/test/material/tabs_test.dart

@@ -3818,6 +3818,68 @@ void main() {
     ));
   });
 
+  testWidgets('TabController changes while flinging', (WidgetTester tester) async {
+    // This is a regression test for https://github.com/flutter/flutter/issues/34744
+
+    Widget buildFrame(TabController controller) {
+
+      return MaterialApp(
+        theme: ThemeData(platform: TargetPlatform.iOS),
+        home: Scaffold(
+          appBar: AppBar(
+            title: const Text('tabs'),
+            bottom: TabBar(
+              controller: controller,
+              tabs: <Tab>[
+                const Tab(text: 'A'),
+                const Tab(text: 'B'),
+                if (controller.length == 3)
+                  const Tab(text: 'C'),
+              ],
+            ),
+          ),
+          body: TabBarView(
+            controller: controller,
+            children: <Widget>[
+              const Center(child: Text('CHILD A')),
+              const Center(child: Text('CHILD B')),
+              if (controller.length == 3)
+                const Center(child: Text('CHILD C')),
+            ],
+          ),
+        ),
+      );
+    }
+
+    final TabController controller1 = TabController(
+      vsync: const TestVSync(),
+      length: 2,
+    );
+
+    final TabController controller2 = TabController(
+      vsync: const TestVSync(),
+      length: 3,
+    );
+
+    expect(controller1.index, 0);
+    expect(controller2.index, 0);
+
+    await tester.pumpWidget(buildFrame(controller1));
+    final Offset flingStart = tester.getCenter(find.text('CHILD A'));
+    await tester.flingFrom(flingStart, const Offset(-200.0, 0.0), 10000.0);
+    await tester.pump(const Duration(milliseconds: 10)); // start the fling animation
+
+    controller1.dispose();
+    await tester.pump(const Duration(milliseconds: 10));
+
+    await tester.pumpWidget(buildFrame(controller2)); // replace controller
+    await tester.flingFrom(flingStart, const Offset(-200.0, 0.0), 10000.0);
+    await tester.pumpAndSettle(); // finish the fling animation
+
+    expect(controller1.index, 0);
+    expect(controller2.index, 1);
+  });
+
   testWidgets('TabController changes with different initialIndex', (WidgetTester tester) async {
     // This is a regression test for https://github.com/flutter/flutter/issues/115917
     const Key lastTabKey = Key('Last Tab');