fix: fix event.target still can be pointed by event after finalized by JavaScript GC. (#519)

<img width="901" alt="image"
src="https://github.com/openwebf/webf/assets/4409743/aaf69f14-965e-4cb2-b8a1-1933e87ed8c5">

Author: andycall

webf/lib/src/bridge/binding.dart

@@ -49,7 +49,12 @@ void _dispatchEventToNative(Event event, bool isCapture) {
   Pointer<NativeBindingObject>? pointer = event.currentTarget?.pointer;
   int? contextId = event.target?.contextId;
   WebFController controller = WebFController.getControllerOfJSContextId(contextId)!;
-  if (contextId != null && pointer != null && pointer.ref.invokeBindingMethodFromDart != nullptr) {
+  if (contextId != null &&
+      pointer != null &&
+      pointer.ref.invokeBindingMethodFromDart != nullptr &&
+      event.target?.pointer?.ref.disposed != true &&
+      event.currentTarget?.pointer?.ref.disposed != true
+  ) {
     BindingObject bindingObject = controller.view.getBindingObject(pointer);
     // Call methods implements at C++ side.
     DartInvokeBindingMethodsFromDart f = pointer.ref.invokeBindingMethodFromDart.asFunction();