Fix null check crash by ReorderableList (flutter#132153)

Fix issue where if you drag the last element of `ReorderableList` and put it in the same index, a `Null check` error arises. This happens as index in `_items` is out of bounds (when `reverse: true`). Fix is to check if last element, dragged element and drop index is same, and return as nothing has changed. Find this video attached.

https://github.com/flutter/flutter/assets/84124091/8043cac3-eb08-42e1-87e7-8095ecab09dc

Fixes issue flutter#132077

Author: opxdelwin

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

@@ -798,6 +798,25 @@ class SliverReorderableListState extends State<SliverReorderableList> with Ticke
   }
 
   void _dragEnd(_DragInfo item) {
+    // No changes required if last child is being inserted into the last position.
+    if ((_insertIndex! + 1 == _items.length) && _reverse) {
+      final RenderBox lastItemRenderBox =  _items[_items.length - 1]!.context.findRenderObject()! as RenderBox;
+      final Offset lastItemOffset =  lastItemRenderBox.localToGlobal(Offset.zero);
+
+      // When drag starts, the corresponding element is removed from
+      // the list, and moves inside of [ReorderableListState.CustomScrollView],
+      // which gives [CustomScrollView] a variable height.
+      //
+      // So when the element is moved, delta would change accordingly,
+      // and since it's the last element,
+      // we animate it back to it's position and add it back to the list.
+      final double delta = item.itemSize.height;
+
+      setState(() {
+        _finalDropPosition = Offset(lastItemOffset.dx, lastItemOffset.dy  - delta);
+      });
+      return;
+    }
     setState(() {
       if (_insertIndex == item.index) {
         _finalDropPosition = _itemOffsetAt(_insertIndex! + (_reverse ? 1 : 0));

packages/flutter/test/widgets/reorderable_list_test.dart

@@ -1309,6 +1309,51 @@ void main() {
 
     expect(offsetForFastScroller / offsetForSlowScroller, fastVelocityScalar / slowVelocityScalar);
   });
+
+  testWidgets('Null check error when dragging and dropping last element into last index with reverse:true', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/132077
+    const int itemCount = 5;
+    final List<String> items = List<String>.generate(itemCount, (int index) => 'Item ${index+1}');
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: ReorderableList(
+          onReorder: (int oldIndex, int newIndex) {
+                if (newIndex > oldIndex) {
+                  newIndex -= 1;
+                }
+                final String item = items.removeAt(oldIndex);
+                items.insert(newIndex, item);
+          },
+          itemCount: items.length,
+          reverse: true,
+          itemBuilder: (BuildContext context, int index) {
+            return ReorderableDragStartListener(
+              key: Key('$index'),
+              index: index,
+              child: Material(
+                child: ListTile(
+                  title: Text(items[index]),
+                ),
+              ),
+            );
+          },
+        ),
+      )
+    );
+
+    // Start gesture on last item
+    final TestGesture drag = await tester.startGesture(tester.getCenter(find.text('Item 5')));
+    await tester.pump(kLongPressTimeout);
+
+    // Drag to move up the last item, and drop at the last index
+    await drag.moveBy(const Offset(0, -50));
+    await tester.pump();
+    await drag.up();
+    await tester.pumpAndSettle();
+
+    expect(tester.takeException(), null);
+  });
 }
 
 class TestList extends StatelessWidget {