Fix TabBar crash with SliverAppBar (flutter#154485)

Fixes flutter#154484 

Similar to flutter#104998. Cause with updated at different phases of the same frame.

Author: hgraceb

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

@@ -219,7 +219,7 @@ class ReorderableListView extends StatefulWidget {
   ///
   ///
   /// ** See code in examples/api/lib/material/reorderable_list/reorderable_list_view.build_default_drag_handles.0.dart **
-  ///{@end-tool}
+  /// {@end-tool}
   final bool buildDefaultDragHandles;
 
   /// {@macro flutter.widgets.reorderable_list.padding}

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

@@ -1742,9 +1742,11 @@ class _TabBarState extends State<TabBar> {
         wrappedTabs[previousIndex] = _buildStyledTab(wrappedTabs[previousIndex], false, animation, _defaults);
       } else {
         // The user is dragging the TabBarView's PageView left or right.
-        final int tabIndex = _currentIndex!;
-        final Animation<double> centerAnimation = _DragAnimation(_controller!, tabIndex);
-        wrappedTabs[tabIndex] = _buildStyledTab(wrappedTabs[tabIndex], true, centerAnimation, _defaults);
+        if (_currentIndex! < widget.tabs.length) {
+          final int tabIndex = _currentIndex!;
+          final Animation<double> centerAnimation = _DragAnimation(_controller!, tabIndex);
+          wrappedTabs[tabIndex] = _buildStyledTab(wrappedTabs[tabIndex], true, centerAnimation, _defaults);
+        }
         if (_currentIndex! > 0) {
           final int tabIndex = _currentIndex! - 1;
           final Animation<double> previousAnimation = ReverseAnimation(_DragAnimation(_controller!, tabIndex));

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

@@ -1618,10 +1618,10 @@ class EditableText extends StatefulWidget {
   /// {@endtemplate}
   final bool cursorOpacityAnimates;
 
-  ///{@macro flutter.rendering.RenderEditable.cursorOffset}
+  /// {@macro flutter.rendering.RenderEditable.cursorOffset}
   final Offset? cursorOffset;
 
-  ///{@macro flutter.rendering.RenderEditable.paintCursorAboveText}
+  /// {@macro flutter.rendering.RenderEditable.paintCursorAboveText}
   final bool paintCursorAboveText;
 
   /// Controls how tall the selection highlight boxes are computed to be.

packages/flutter/test/material/tabs_test.dart

@@ -5947,6 +5947,55 @@ void main() {
     gesture.removePointer();
   });
 
+  testWidgets('No crash if TabBar build called before didUpdateWidget with SliverAppBar', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/154484.
+    final List<String> tabs = <String>[];
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: StatefulBuilder(
+          builder: (BuildContext context, StateSetter setState) {
+            return DefaultTabController(
+              length: tabs.length,
+              child: Scaffold(
+                body: CustomScrollView(
+                  slivers: <Widget>[
+                    SliverAppBar(
+                      actions: <Widget>[
+                        TextButton(
+                          child: const Text('Add Tab'),
+                          onPressed: () {
+                            setState(() {
+                              tabs.add('Tab ${tabs.length + 1}');
+                            });
+                          },
+                        ),
+                      ],
+                      bottom: TabBar(
+                        tabs: tabs.map((String tab) => Tab(text: tab)).toList(),
+                      ),
+                    ),
+                  ],
+                ),
+              ),
+            );
+          },
+        ),
+      ),
+    );
+
+    // Initializes with zero tabs.
+    expect(find.text('Tab 1'), findsNothing);
+    expect(find.text('Tab 2'), findsNothing);
+
+    // No crash after tabs added.
+    await tester.tap(find.text('Add Tab'));
+    await tester.pumpAndSettle();
+    expect(find.text('Tab 1'), findsOneWidget);
+    expect(find.text('Tab 2'), findsNothing);
+    expect(tester.takeException(), isNull);
+  });
+
   testWidgets('Do not crash if the controller and TabBarView are updated at different phases(build and layout) of the same frame', (WidgetTester tester) async {
     // Regression test for https://github.com/flutter/flutter/issues/104994.
     List<String> tabTextContent = <String>[];