Allow nullable tweens in TweenAnimationBuilder (#72476)

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

@@ -113,7 +113,7 @@ import 'value_listenable_builder.dart';
 /// [AnimatedBuilder], which can be used similarly to this
 /// [TweenAnimationBuilder], but unlike the latter it is powered by a
 /// developer-managed [AnimationController].
-class TweenAnimationBuilder<T extends Object> extends ImplicitlyAnimatedWidget {
+class TweenAnimationBuilder<T extends Object?> extends ImplicitlyAnimatedWidget {
   /// Creates a [TweenAnimationBuilder].
   ///
   /// The properties [tween], [duration], and [builder] are required. The values
@@ -194,7 +194,7 @@ class TweenAnimationBuilder<T extends Object> extends ImplicitlyAnimatedWidget {
   }
 }
 
-class _TweenAnimationBuilderState<T extends Object> extends AnimatedWidgetBaseState<TweenAnimationBuilder<T>> {
+class _TweenAnimationBuilderState<T extends Object?> extends AnimatedWidgetBaseState<TweenAnimationBuilder<T>> {
   Tween<T>? _currentTween;
 
   @override

packages/flutter/test/widgets/tween_animation_builder_test.dart

@@ -387,4 +387,21 @@ void main() {
     await tester.pump(const Duration(seconds: 2));
     expect(values, everyElement(300));
   });
+
+  testWidgets('Works with nullable tweens', (WidgetTester tester) async {
+    final List<Size?> values = <Size?>[];
+    await tester.pumpWidget(
+      TweenAnimationBuilder<Size?>(
+        duration: const Duration(seconds: 1),
+        tween: SizeTween(end: const Size(10,10)),
+        builder: (BuildContext context, Size? s, Widget? child) {
+          values.add(s);
+          return const Placeholder();
+        },
+      ),
+    );
+    expect(values, <Size>[const Size(10,10)]);
+    await tester.pump(const Duration(seconds: 2));
+    expect(values, <Size>[const Size(10,10)]);
+  });
 }