pass controller in focus callback

Author: hellohuanlin

packages/flutter/lib/src/widgets/platform_view.dart

@@ -569,12 +569,13 @@ class _UiKitViewState extends State<UiKitView> {
 
   @override
   Widget build(BuildContext context) {
-    if (_controller == null) {
+    final UiKitViewController? controller = _controller;
+    if (controller == null) {
       return const SizedBox.expand();
     }
     return Focus(
       focusNode: _focusNode,
-      onFocusChange: _onFocusChange,
+      onFocusChange: (bool isFocused) => _onFocusChange(isFocused, controller),
       child: _UiKitPlatformView(
         controller: _controller!,
         hitTestBehavior: widget.hitTestBehavior,
@@ -659,12 +660,7 @@ class _UiKitViewState extends State<UiKitView> {
     });
   }
 
-  void _onFocusChange(bool isFocused) {
-    assert(_controller != null, 'warning: attempt to focus a UIKitView when it is not ready. ');
-    if (_controller == null) {
-      return;
-    }
-
+  void _onFocusChange(bool isFocused, UiKitViewController controller) {
     if (!isFocused) {
       // Unlike Android, we do not need to send "clearFocus" channel message
       // to the engine, because focusing on another view will automatically
@@ -673,7 +669,7 @@ class _UiKitViewState extends State<UiKitView> {
     }
     SystemChannels.textInput.invokeMethod<void>(
       'TextInput.setPlatformViewClient',
-      <String, dynamic>{'platformViewId': _controller!.id},
+      <String, dynamic>{'platformViewId': controller.id},
     );
   }
 }