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

fixes flutter/flutter#134854

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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');