fix DropdownMenu keyboard navigation when entries are filtered empty (#155252)

fixes flutter/flutter#154532

Author: PurplePolyhedron

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

@@ -807,6 +807,7 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
     } else {
       filteredEntries = widget.dropdownMenuEntries;
     }
+    _menuHasEnabledItem = filteredEntries.any((DropdownMenuEntry<T> entry) => entry.enabled);
 
     if (_enableSearch) {
       if (widget.searchCallback != null) {

packages/flutter/test/material/dropdown_menu_test.dart

@@ -1315,6 +1315,28 @@ void main() {
     expect(tester.takeException(), isNull);
   });
 
+  // Regression test for https://github.com/flutter/flutter/issues/154532.
+  testWidgets('Searching for non matching item does not crash', (WidgetTester tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: Scaffold(
+        body: DropdownMenu<TestMenu>(
+          enableFilter: true,
+          requestFocusOnTap: true,
+          dropdownMenuEntries: menuChildren,
+        ),
+      ),
+    ));
+
+    // Open the menu.
+    await tester.tap(find.byType(DropdownMenu<TestMenu>));
+    await tester.pump();
+    await tester.enterText(find.byType(TextField).first, 'Me');
+    await tester.pumpAndSettle();
+    await tester.enterText(find.byType(TextField).first, 'Meu');
+    await tester.pumpAndSettle();
+    expect(tester.takeException(), isNull);
+  });
+
   // Regression test for https://github.com/flutter/flutter/issues/147253.
   testWidgets('Default search prioritises the current highlight on desktop platforms',
       (WidgetTester tester) async {