Handle privatecommand messages that pass no data (#112590)

Fixes an error in some text editors.

packages/flutter/lib/src/services/text_input.dart

@@ -1877,7 +1877,9 @@ class TextInput {
         final Map<String, dynamic> firstArg = args[1] as Map<String, dynamic>;
         _currentConnection!._client.performPrivateCommand(
           firstArg['action'] as String,
-          firstArg['data'] as Map<String, dynamic>,
+          firstArg['data'] == null
+              ? <String, dynamic>{}
+              : firstArg['data'] as Map<String, dynamic>,
         );
         break;
       case 'TextInputClient.updateFloatingCursor':

packages/flutter/test/services/text_input_test.dart

@@ -562,6 +562,32 @@ void main() {
       expect(client.latestMethodCall, 'performPrivateCommand');
     });
 
+    test('TextInputClient performPrivateCommand method is called with no data at all', () async {
+      // Assemble a TextInputConnection so we can verify its change in state.
+      final FakeTextInputClient client = FakeTextInputClient(TextEditingValue.empty);
+      const TextInputConfiguration configuration = TextInputConfiguration();
+      TextInput.attach(client, configuration);
+
+      expect(client.latestMethodCall, isEmpty);
+
+      // Send performPrivateCommand message.
+      final ByteData? messageBytes = const JSONMessageCodec().encodeMessage(<String, dynamic>{
+        'args': <dynamic>[
+          1,
+          jsonDecode('{"action": "actionCommand"}'), // No `data` parameter.
+        ],
+        'method': 'TextInputClient.performPrivateCommand',
+      });
+      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+        'flutter/textinput',
+        messageBytes,
+        (ByteData? _) {},
+      );
+
+      expect(client.latestMethodCall, 'performPrivateCommand');
+      expect(client.latestPrivateCommandData, <String, dynamic>{});
+    });
+
     test('TextInputClient showAutocorrectionPromptRect method is called', () async {
       // Assemble a TextInputConnection so we can verify its change in state.
       final FakeTextInputClient client = FakeTextInputClient(TextEditingValue.empty);
@@ -933,6 +959,7 @@ class FakeTextInputClient with TextInputClient {
 
   String latestMethodCall = '';
   final List<String> performedSelectors = <String>[];
+  late Map<String, dynamic>? latestPrivateCommandData;
 
   @override
   TextEditingValue currentTextEditingValue;
@@ -946,8 +973,9 @@ class FakeTextInputClient with TextInputClient {
   }
 
   @override
-  void performPrivateCommand(String action, Map<String, dynamic> data) {
+  void performPrivateCommand(String action, Map<String, dynamic>? data) {
     latestMethodCall = 'performPrivateCommand';
+    latestPrivateCommandData = data;
   }
 
   @override