change LoadEnvelope method

EnvelopeGenerator.Application/Extensions/TaskExtensions.cs

@@ -36,4 +36,18 @@ public static class TaskExtensions
         var result = await task;
         return result?.Any() ?? false ? result : throw new NotFoundException(exceptionMessage);
     }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <typeparam name="T"></typeparam>
+    /// <typeparam name="I"></typeparam>
+    /// <param name="task"></param>
+    /// <param name="act"></param>
+    /// <returns></returns>
+    public static async Task<I> Then<T, I>(this Task<T> task, Func<T, I> act)
+    {
+        var res = await task;
+        return act(res);
+    }
 }