Fix ThemeData extension throws when the ThemeExtension not found (#103343)

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

@@ -1077,7 +1077,7 @@ class ThemeData with Diagnosticable {
   /// Obtain with `Theme.of(context).extension<MyThemeExtension>()`.
   ///
   /// See [extensions] for an interactive example.
-  T? extension<T>() => extensions[T] as T;
+  T? extension<T>() => extensions[T] as T?;
 
   /// The default [InputDecoration] values for [InputDecorator], [TextField],
   /// and [TextFormField] are based on this theme.

packages/flutter/test/material/theme_data_test.dart

@@ -561,6 +561,14 @@ void main() {
       expect(lerped.extension<MyThemeExtensionA>()!.color2, const Color(0xff90ab7d));
       expect(lerped.extension<MyThemeExtensionB>()!.textStyle, const TextStyle(fontSize: 100)); // Not lerped
     });
+
+    testWidgets('should return null on extension not found', (WidgetTester tester) async {
+      final ThemeData theme = ThemeData(
+        extensions: const <ThemeExtension<dynamic>>{},
+      );
+
+      expect(theme.extension<MyThemeExtensionA>(), isNull);
+    });
   });
 
   test('copyWith, ==, hashCode basics', () {