fix: avoiding trigger touch events to inaccessible dom elements. (#517)

The target element of a Touch object may be reclaimed by the JavaScript
garbage collector, causing it to become invisible and inaccessible.

This update introduces checks to prevent the triggering of events on
these invisible DOM targets.


<img width="892" alt="image"
src="https://github.com/openwebf/webf/assets/4409743/4db7d025-3114-4d5a-8587-31cfa7f06ae6">

webf/lib/src/gesture/gesture_dispatcher.dart

@@ -4,6 +4,7 @@
  */
 
 import 'dart:async';
+import 'dart:ffi';
 
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
@@ -382,6 +383,13 @@ class GestureDispatcher {
         TouchPoint touchPoint = touchPoints[i];
         Touch touch = _toTouch(touchPoint);
 
+        // The touch target might be eliminated from the DOM tree and collected by JavaScript GC,
+        // resulting in it becoming invisible and inaccessible, yet this change is not synchronized with Dart instantly.
+        // Therefore, refrain from triggering events on these unavailable DOM targets.
+        if (touch.target.pointer?.ref.disposed == true) {
+          continue;
+        }
+
         if (currentTouchPoint.id == touchPoint.id) {
           // TODO: add pointEvent list for handle pointEvent at the current frame and support changedTouches.
           e.changedTouches.append(touch);
@@ -394,7 +402,9 @@ class GestureDispatcher {
         e.touches.append(touch);
       }
 
-      _pointTargets[currentTouchPoint.id]?.dispatchEvent(e);
+      if (e.touches.length > 0) {
+        _pointTargets[currentTouchPoint.id]?.dispatchEvent(e);
+      }
     }
   }
 }