Fixes focus traversal crash if the current node can't request focus (flutter#134954)

fixes flutter#134854

## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [ ] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [ ] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

Author: chunhtai

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

@@ -433,7 +433,9 @@ abstract class FocusTraversalPolicy with Diagnosticable {
     // finds.
     assert((){
       final Set<FocusNode> difference = sortedDescendants.toSet().difference(scope.traversalDescendants.toSet());
-      if (currentNode.skipTraversal) {
+      if (currentNode.skipTraversal || !currentNode.canRequestFocus) {
+        // The scope.traversalDescendants will not contain currentNode if it
+        // skips traversal or not focusable.
         assert(
          difference.length == 1 && difference.contains(currentNode),
          'Sorted descendants contains different nodes than FocusScopeNode.traversalDescendants would. '

packages/flutter/test/widgets/focus_traversal_test.dart

@@ -596,6 +596,41 @@ void main() {
       expect(scope.hasFocus, isTrue);
     });
 
+    testWidgetsWithLeakTracking('Requesting nextFocus on node focuses its descendant', (WidgetTester tester) async {
+      for (final bool canRequestFocus in <bool>{true, false}) {
+        final FocusNode node1 = FocusNode();
+        final FocusNode node2 = FocusNode();
+        addTearDown(() {
+          node1.dispose();
+          node2.dispose();
+        });
+        await tester.pumpWidget(
+          Directionality(
+            textDirection: TextDirection.ltr,
+            child: FocusTraversalGroup(
+              policy: ReadingOrderTraversalPolicy(),
+              child: FocusScope(
+                child: Focus(
+                  focusNode: node1,
+                  canRequestFocus: canRequestFocus,
+                  child: Focus(
+                    focusNode: node2,
+                    child: Container(),
+                  ),
+                ),
+              ),
+            ),
+          ),
+        );
+
+        final bool didFindNode = node1.nextFocus();
+        await tester.pump();
+        expect(didFindNode, isTrue);
+        expect(node1.hasPrimaryFocus, isFalse);
+        expect(node2.hasPrimaryFocus, isTrue);
+      }
+    });
+
     testWidgetsWithLeakTracking('Move reading focus to previous node.', (WidgetTester tester) async {
       final GlobalKey key1 = GlobalKey(debugLabel: '1');
       final GlobalKey key2 = GlobalKey(debugLabel: '2');