showOnScreen does not crash if target node doesn't exist anymore (#121575)

showOnScreen does not crash if target node doesn't exist anymore

packages/flutter/lib/src/semantics/semantics.dart

@@ -3248,7 +3248,7 @@ class SemanticsOwner extends ChangeNotifier {
     }
 
     // Default actions if no [handler] was provided.
-    if (action == SemanticsAction.showOnScreen && _nodes[id]!._showOnScreen != null) {
+    if (action == SemanticsAction.showOnScreen && _nodes[id]?._showOnScreen != null) {
       _nodes[id]!._showOnScreen!();
     }
   }

packages/flutter/test/semantics/semantics_owner_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter/semantics.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+import '../widgets/semantics_tester.dart';
+
+void main() {
+  testWidgets('Performing SemanticsAction.showOnScreen does not crash if node no longer exist', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/100358.
+
+    final SemanticsTester semantics = SemanticsTester(tester);
+    addTearDown(semantics.dispose);
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Semantics(
+          explicitChildNodes: true,
+          child: const Text('Hello World'),
+        ),
+      ),
+    );
+
+    final int nodeId = tester.semantics.find(find.bySemanticsLabel('Hello World')).id;
+
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: Container(),
+      ),
+    );
+
+    // Node with $nodeId is no longer in the tree.
+    expect(semantics, isNot(hasSemantics(TestSemantics(id: nodeId))));
+
+    // Executing SemanticsAction.showOnScreen on that node does not crash.
+    // (A platform may not have processed the semantics update yet and send
+    // actions for no longer existing nodes.)
+    tester.binding.performSemanticsAction(SemanticsActionEvent(
+      type: SemanticsAction.showOnScreen,
+      nodeId: nodeId,
+    ));
+  });
+}