Fix a crash when disposing tabs (#63142)

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

@@ -220,8 +220,10 @@ class TabController extends ChangeNotifier {
       _animationController
         .animateTo(_index.toDouble(), duration: duration, curve: curve)
         .whenCompleteOrCancel(() {
-          _indexIsChangingCount -= 1;
-          notifyListeners();
+          if (_animationController != null) { // don't notify if we've been disposed
+            _indexIsChangingCount -= 1;
+            notifyListeners();
+          }
         });
     } else {
       _indexIsChangingCount += 1;

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

@@ -1199,8 +1199,6 @@ class TabBarView extends StatefulWidget {
   _TabBarViewState createState() => _TabBarViewState();
 }
 
-const PageScrollPhysics _kTabBarViewPhysics = PageScrollPhysics();
-
 class _TabBarViewState extends State<TabBarView> {
   TabController _controller;
   PageController _pageController;
@@ -1370,8 +1368,8 @@ class _TabBarViewState extends State<TabBarView> {
         dragStartBehavior: widget.dragStartBehavior,
         controller: _pageController,
         physics: widget.physics == null
-          ? _kTabBarViewPhysics.applyTo(const ClampingScrollPhysics())
-          : _kTabBarViewPhysics.applyTo(widget.physics),
+          ? const PageScrollPhysics().applyTo(const ClampingScrollPhysics())
+          : const PageScrollPhysics().applyTo(widget.physics),
         children: _childrenWithKey,
       ),
     );

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

@@ -428,8 +428,9 @@ class RangeMaintainingScrollPhysics extends ScrollPhysics {
     @required bool isScrolling,
     @required double velocity,
   }) {
-    if (velocity != 0.0 || ((oldPosition.minScrollExtent == newPosition.minScrollExtent) && (oldPosition.maxScrollExtent == newPosition.maxScrollExtent)))
+    if (velocity != 0.0 || ((oldPosition.minScrollExtent == newPosition.minScrollExtent) && (oldPosition.maxScrollExtent == newPosition.maxScrollExtent))) {
       return super.adjustPositionForNewDimensions(oldPosition: oldPosition, newPosition: newPosition, isScrolling: isScrolling, velocity: velocity);
+    }
     if (oldPosition.pixels < oldPosition.minScrollExtent) {
       final double oldDelta = oldPosition.minScrollExtent - oldPosition.pixels;
       return newPosition.minScrollExtent - oldDelta;

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

@@ -432,55 +432,6 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
     return true;
   }
 
-  Set<SemanticsAction> _semanticActions;
-
-  /// Called whenever the scroll position or the dimensions of the scroll view
-  /// change to schedule an update of the available semantics actions. The
-  /// actual update will be performed in the next frame. If non is pending
-  /// a frame will be scheduled.
-  ///
-  /// For example: If the scroll view has been scrolled all the way to the top,
-  /// the action to scroll further up needs to be removed as the scroll view
-  /// cannot be scrolled in that direction anymore.
-  ///
-  /// This method is potentially called twice per frame (if scroll position and
-  /// scroll view dimensions both change) and therefore shouldn't do anything
-  /// expensive.
-  void _updateSemanticActions() {
-    SemanticsAction forward;
-    SemanticsAction backward;
-    switch (axisDirection) {
-      case AxisDirection.up:
-        forward = SemanticsAction.scrollDown;
-        backward = SemanticsAction.scrollUp;
-        break;
-      case AxisDirection.right:
-        forward = SemanticsAction.scrollLeft;
-        backward = SemanticsAction.scrollRight;
-        break;
-      case AxisDirection.down:
-        forward = SemanticsAction.scrollUp;
-        backward = SemanticsAction.scrollDown;
-        break;
-      case AxisDirection.left:
-        forward = SemanticsAction.scrollRight;
-        backward = SemanticsAction.scrollLeft;
-        break;
-    }
-
-    final Set<SemanticsAction> actions = <SemanticsAction>{};
-    if (pixels > minScrollExtent)
-      actions.add(backward);
-    if (pixels < maxScrollExtent)
-      actions.add(forward);
-
-    if (setEquals<SemanticsAction>(actions, _semanticActions))
-      return;
-
-    _semanticActions = actions;
-    context.setSemanticsActions(_semanticActions);
-  }
-
   @override
   bool applyContentDimensions(double minScrollExtent, double maxScrollExtent) {
     assert(minScrollExtent != null);
@@ -560,6 +511,55 @@ abstract class ScrollPosition extends ViewportOffset with ScrollMetrics {
     _updateSemanticActions(); // will potentially request a semantics update.
   }
 
+  Set<SemanticsAction> _semanticActions;
+
+  /// Called whenever the scroll position or the dimensions of the scroll view
+  /// change to schedule an update of the available semantics actions. The
+  /// actual update will be performed in the next frame. If non is pending
+  /// a frame will be scheduled.
+  ///
+  /// For example: If the scroll view has been scrolled all the way to the top,
+  /// the action to scroll further up needs to be removed as the scroll view
+  /// cannot be scrolled in that direction anymore.
+  ///
+  /// This method is potentially called twice per frame (if scroll position and
+  /// scroll view dimensions both change) and therefore shouldn't do anything
+  /// expensive.
+  void _updateSemanticActions() {
+    SemanticsAction forward;
+    SemanticsAction backward;
+    switch (axisDirection) {
+      case AxisDirection.up:
+        forward = SemanticsAction.scrollDown;
+        backward = SemanticsAction.scrollUp;
+        break;
+      case AxisDirection.right:
+        forward = SemanticsAction.scrollLeft;
+        backward = SemanticsAction.scrollRight;
+        break;
+      case AxisDirection.down:
+        forward = SemanticsAction.scrollUp;
+        backward = SemanticsAction.scrollDown;
+        break;
+      case AxisDirection.left:
+        forward = SemanticsAction.scrollRight;
+        backward = SemanticsAction.scrollLeft;
+        break;
+    }
+
+    final Set<SemanticsAction> actions = <SemanticsAction>{};
+    if (pixels > minScrollExtent)
+      actions.add(backward);
+    if (pixels < maxScrollExtent)
+      actions.add(forward);
+
+    if (setEquals<SemanticsAction>(actions, _semanticActions))
+      return;
+
+    _semanticActions = actions;
+    context.setSemanticsActions(_semanticActions);
+  }
+
   /// Animates the position such that the given object is as visible as possible
   /// by just scrolling this position.
   ///

packages/flutter/test/material/tabs_test.dart

@@ -2670,6 +2670,13 @@ void main() {
     final PageView pageView = tester.widget<PageView>(find.byType(PageView));
     expect(pageView.physics.toString().contains('ClampingScrollPhysics'), isFalse);
   });
+
+  testWidgets('Crash on dispose', (WidgetTester tester) async {
+    await tester.pumpWidget(Padding(padding: const EdgeInsets.only(right: 200.0), child: TabBarDemo()));
+    await tester.tap(find.byIcon(Icons.directions_bike));
+    // There was a time where this would throw an exception
+    // because we tried to send a notification on dispose.
+  });
 }
 
 class KeepAliveInk extends StatefulWidget {
@@ -2693,3 +2700,33 @@ class _KeepAliveInkState extends State<KeepAliveInk> with AutomaticKeepAliveClie
   @override
   bool get wantKeepAlive => true;
 }
+
+class TabBarDemo extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: DefaultTabController(
+        length: 3,
+        child: Scaffold(
+          appBar: AppBar(
+            bottom: const TabBar(
+              tabs: <Widget>[
+                Tab(icon: Icon(Icons.directions_car)),
+                Tab(icon: Icon(Icons.directions_transit)),
+                Tab(icon: Icon(Icons.directions_bike)),
+              ],
+            ),
+            title: const Text('Tabs Demo'),
+          ),
+          body: const TabBarView(
+            children: <Widget>[
+              Icon(Icons.directions_car),
+              Icon(Icons.directions_transit),
+              Icon(Icons.directions_bike),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}