Bugfix/handle empty stack (#4549)

* Added a check for count before poping from stack.

* Fixed test to reflect actual expected exception.

Author: ujwalakhaire

CSharp/Library/Microsoft.Bot.Builder/Fibers/Fiber.cs

@@ -234,7 +234,11 @@ async Task<IWait<C>> IFiberLoop<C>.PollAsync(C context, CancellationToken token)
                 }
                 catch (Exception error)
                 {
-                    this.stack.Pop();
+                    if (this.stack.Count != 0)
+                    {
+                        this.stack.Pop();
+                    }
+
                     if (this.stack.Count == 0)
                     {
                         throw;

CSharp/Tests/Microsoft.Bot.Builder.Tests/FiberTests.cs

@@ -330,7 +330,7 @@ public async Task Fiber_OneCall_NeedDone()
         }
 
         [TestMethod]
-        [ExpectedException(typeof(InvalidOperationException))]
+        [ExpectedException(typeof(InvalidNeedException))]
         public async Task Fiber_OneCall_ThenDone_Throws()
         {
             // arrange