[web] Fix exception when using a keyboard (flutter/engine#24150)

engine/src/flutter/lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -194,10 +194,15 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
   /// Engine code should use this method instead of the callback directly.
   /// Otherwise zones won't work properly.
   void invokeOnKeyData(ui.KeyData data, _KeyDataResponseCallback callback) {
-    invoke(
-      () { callback(onKeyData == null ? false : onKeyData!(data)); },
-      _onKeyDataZone,
-    );
+    final ui.KeyDataCallback? onKeyData = _onKeyData;
+    if (onKeyData != null) {
+      invoke(
+        () => callback(onKeyData(data)),
+        _onKeyDataZone,
+      );
+    } else {
+      callback(false);
+    }
   }
 
   /// A callback that is invoked to report the [FrameTiming] of recently

engine/src/flutter/lib/web_ui/test/engine/window_test.dart

@@ -13,6 +13,9 @@ import 'package:test/test.dart';
 import 'package:ui/ui.dart' as ui;
 import 'package:ui/src/engine.dart';
 
+const int kPhysicalKeyA = 0x00070004;
+const int kLogicalKeyA = 0x00000000061;
+
 void main() {
   internalBootstrapBrowserTest(() => testMain);
 }
@@ -146,6 +149,52 @@ void testMain() {
     EnginePlatformDispatcher.instance.invokeOnPointerDataPacket(null);
   });
 
+  test('invokeOnKeyData returns normally when onKeyData is null', () {
+    final ui.KeyData keyData = ui.KeyData(
+      timeStamp: Duration(milliseconds: 1),
+      type: ui.KeyEventType.repeat,
+      physical: kPhysicalKeyA,
+      logical: kLogicalKeyA,
+      character: 'a',
+      synthesized: true,
+    );
+    expect(() {
+      EnginePlatformDispatcher.instance.invokeOnKeyData(keyData, (bool result) {
+        expect(result, isFalse);
+      });
+    }, returnsNormally);
+  });
+
+  test('onKeyData preserves the zone', () {
+    final Zone innerZone = Zone.current.fork();
+
+    innerZone.runGuarded(() {
+      final ui.KeyDataCallback onKeyData = (_) {
+        expect(Zone.current, innerZone);
+        return false;
+      };
+      window.onKeyData = onKeyData;
+
+      // Test that the getter returns the exact same onKeyData, e.g. it doesn't
+      // wrap it.
+      expect(window.onKeyData, same(onKeyData));
+    });
+
+    final ui.KeyData keyData = ui.KeyData(
+      timeStamp: Duration(milliseconds: 1),
+      type: ui.KeyEventType.repeat,
+      physical: kPhysicalKeyA,
+      logical: kLogicalKeyA,
+      character: 'a',
+      synthesized: true,
+    );
+    EnginePlatformDispatcher.instance.invokeOnKeyData(keyData, (bool result) {
+      expect(result, isFalse);
+    });
+
+    window.onKeyData = null;
+  });
+
   test('onSemanticsEnabledChanged preserves the zone', () {
     final Zone innerZone = Zone.current.fork();