Update Operation System

Assets/YooAsset/Runtime/OperationSystem/AsyncOperationBase.cs

@@ -81,6 +81,14 @@ namespace YooAsset
 				_taskCompletionSource.TrySetResult(null);
 		}
 
+		/// <summary>
+		/// 清空完成回调
+		/// </summary>
+		protected void ClearCompletedCallback()
+		{
+			_callback = null;
+		}
+
 		#region 异步编程相关
 		bool IEnumerator.MoveNext()
 		{