Fixed #26

修复多个场景打进一个AB包时，卸载子场景时抛出异常。

Assets/YooAsset/Runtime/AssetSystem/Provider/ProviderBase.cs

@@ -17,6 +17,11 @@ namespace YooAsset
 			Fail,
 		}
 
+		/// <summary>
+		/// 资源提供者唯一标识符
+		/// </summary>
+		public string ProviderGUID { private set; get; }
+
 		/// <summary>
 		/// 资源信息
 		/// </summary>
@@ -85,8 +90,9 @@ namespace YooAsset
 		private readonly List<OperationHandleBase> _handles = new List<OperationHandleBase>();
 
 
-		public ProviderBase(AssetInfo assetInfo)
+		public ProviderBase(string providerGUID, AssetInfo assetInfo)
 		{
+			ProviderGUID = providerGUID;
 			MainAssetInfo = assetInfo;
 		}