fix: Dropdown menu trying to access highlight element which doesn't exist when search and filters both are enabled (flutter#151969)

DropdownMenu throws RangeError when both filter and search are enabled because when we search for elements, we have some highlighted element, but if there is no element to highlight it tries to access 0th element is the filtered entries, but entries are empty.

Fixes flutter#151878

Author: rkishan516

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

@@ -613,14 +613,30 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
       .toList();
   }
 
+  bool _shouldUpdateCurrentHighlight(List<DropdownMenuEntry<T>> entries) {
+    final String searchText = _localTextEditingController!.value.text.toLowerCase();
+    if (searchText.isEmpty) {
+      return true;
+    }
+
+    // When `entries` are filtered by filter algorithm, currentHighlight may exceed the valid range of `entries` and should be updated.
+    if (currentHighlight == null || currentHighlight! >= entries.length) {
+      return true;
+    }
+
+    if (entries[currentHighlight!].label.toLowerCase().contains(searchText)) {
+      return false;
+    }
+
+    return true;
+  }
+
   int? search(List<DropdownMenuEntry<T>> entries, TextEditingController textEditingController) {
     final String searchText = textEditingController.value.text.toLowerCase();
     if (searchText.isEmpty) {
       return null;
     }
-    if (currentHighlight != null && entries[currentHighlight!].label.toLowerCase().contains(searchText)) {
-      return currentHighlight;
-    }
+
     final int index = entries.indexWhere((DropdownMenuEntry<T> entry) => entry.label.toLowerCase().contains(searchText));
 
     return index != -1 ? index : null;
@@ -796,7 +812,10 @@ class _DropdownMenuState<T> extends State<DropdownMenu<T>> {
       if (widget.searchCallback != null) {
         currentHighlight = widget.searchCallback!(filteredEntries, _localTextEditingController!.text);
       } else {
-        currentHighlight = search(filteredEntries, _localTextEditingController!);
+        final bool shouldUpdateCurrentHighlight = _shouldUpdateCurrentHighlight(filteredEntries);
+        if (shouldUpdateCurrentHighlight) {
+          currentHighlight = search(filteredEntries, _localTextEditingController!);
+        }
       }
       if (currentHighlight != null) {
         scrollToHighlight();

packages/flutter/test/material/dropdown_menu_test.dart

@@ -1292,6 +1292,29 @@ void main() {
 
   }, variant: TargetPlatformVariant.desktop());
 
+  // Regression test for https://github.com/flutter/flutter/issues/151878.
+  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 {