update SearchDelegate's leading and actions widgets can be null (#80006)

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

@@ -180,7 +180,7 @@ abstract class SearchDelegate<T> {
   /// See also:
   ///
   ///  * [AppBar.leading], the intended use for the return value of this method.
-  Widget buildLeading(BuildContext context);
+  Widget? buildLeading(BuildContext context);
 
   /// Widgets to display after the search query in the [AppBar].
   ///
@@ -193,7 +193,7 @@ abstract class SearchDelegate<T> {
   /// See also:
   ///
   ///  * [AppBar.actions], the intended use for the return value of this method.
-  List<Widget> buildActions(BuildContext context);
+  List<Widget>? buildActions(BuildContext context);
 
   /// Widget to display across the bottom of the [AppBar].
   ///

packages/flutter/test/material/search_test.dart

@@ -771,6 +771,48 @@ void main() {
       expect(textField.style!.color, themeData.textTheme.bodyText1!.color);
       expect(textField.style!.color, isNot(equals(themeData.primaryColor)));
   });
+
+  // Regression test for: https://github.com/flutter/flutter/issues/78144
+  testWidgets('`Leading` and `Actions` nullable test', (WidgetTester tester) async {
+    // The search delegate page is displayed with no issues
+    // even with a null return values for [buildLeading] and [buildActions].
+    final _TestEmptySearchDelegate delegate = _TestEmptySearchDelegate(
+      result: 'Result',
+      suggestions: 'Suggestions',
+    );
+    final List<String> selectedResults = <String>[];
+
+    await tester.pumpWidget(TestHomePage(
+      delegate: delegate,
+      results: selectedResults,
+    ));
+
+    // We are on the homepage.
+    expect(find.text('HomeBody'), findsOneWidget);
+    expect(find.text('HomeTitle'), findsOneWidget);
+    expect(find.text('Suggestions'), findsNothing);
+
+    // Open the search page.
+    await tester.tap(find.byTooltip('Search'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('HomeBody'), findsNothing);
+    expect(find.text('HomeTitle'), findsNothing);
+    expect(find.text('Suggestions'), findsOneWidget);
+    expect(selectedResults, hasLength(0));
+
+    final TextField textField = tester.widget(find.byType(TextField));
+    expect(textField.focusNode!.hasFocus, isTrue);
+
+    // Close the search page.
+    await tester.tap(find.byTooltip('Close'));
+    await tester.pumpAndSettle();
+
+    expect(find.text('HomeBody'), findsOneWidget);
+    expect(find.text('HomeTitle'), findsOneWidget);
+    expect(find.text('Suggestions'), findsNothing);
+    expect(selectedResults, <String>['Result']);
+  });
 }
 
 class TestHomePage extends StatelessWidget {
@@ -911,3 +953,48 @@ class _TestSearchDelegate extends SearchDelegate<String> {
     );
   }
 }
+
+class _TestEmptySearchDelegate extends SearchDelegate<String> {
+  _TestEmptySearchDelegate({
+    this.suggestions = 'Suggestions',
+    this.result = 'Result',
+  }) : super();
+
+  final String suggestions;
+  final String result;
+
+  @override
+  Widget? buildLeading(BuildContext context) => null;
+
+  @override
+  List<Widget>? buildActions(BuildContext context) => null;
+
+  @override
+  Widget buildSuggestions(BuildContext context) {
+    return MaterialButton(
+      onPressed: () {
+        showResults(context);
+      },
+      child: Text(suggestions),
+    );
+  }
+
+  @override
+  Widget buildResults(BuildContext context) {
+    return const Text('Results');
+  }
+
+  @override
+  PreferredSizeWidget buildBottom(BuildContext context) {
+    return PreferredSize(
+      preferredSize: const Size.fromHeight(56.0),
+      child: IconButton(
+        tooltip: 'Close',
+        icon: const Icon(Icons.arrow_back),
+        onPressed: () {
+          close(context, result);
+        },
+      ),
+    );
+  }
+}