Fix broken test

Author: Leo

FirebaseAdmin/FirebaseAdmin.Tests/Messaging/TopicManagementResponseTest.cs

@@ -15,7 +15,7 @@ public void SuccessfulReponse()
             var response = new TopicManagementResponse(topicManagementResults);
 
             Assert.Empty(response.Errors);
-            Assert.Equal(2, response.SuccessCount);
+            Assert.Equal(1, response.SuccessCount);
         }
 
         [Fact]
@@ -34,7 +34,7 @@ public void UnsuccessfulResponse()
         [Fact]
         public void NullResponse()
         {
-            Assert.Throws<ArgumentException>(() =>
+            Assert.Throws<ArgumentNullException>(() =>
             {
                 new TopicManagementResponse(null);
             });
@@ -55,9 +55,7 @@ public void UnknownError()
             var topicManagementResults = new List<string> { "NOT_A_REAL_ERROR_CODE" };
             var response = new TopicManagementResponse(topicManagementResults);
 
-            Assert.Empty(response.Errors);
-            Assert.Equal(0, response.SuccessCount);
-            Assert.NotEmpty(response.Errors);
+            Assert.Single(response.Errors);
             Assert.Equal("unknown-error", response.Errors[0].Reason);
             Assert.Equal(0, response.Errors[0].Index);
         }
@@ -68,8 +66,9 @@ public void UnexpectedResponse()
             var topicManagementResults = new List<string> { "NOT_A_REAL_CODE" };
             var response = new TopicManagementResponse(topicManagementResults);
 
-            Assert.Empty(response.Errors);
-            Assert.Equal(1, response.SuccessCount);
+            Assert.Single(response.Errors);
+            Assert.Equal("unknown-error", response.Errors[0].Reason);
+            Assert.Equal(0, response.SuccessCount);
         }
 
         [Fact]

FirebaseAdmin/FirebaseAdmin/Messaging/TopicManagementResponse.cs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+using System;
 using System.Collections.Generic;
 using System.Linq;
 
@@ -22,15 +23,21 @@ namespace FirebaseAdmin.Messaging
     /// </summary>
     public sealed class TopicManagementResponse
     {
-        private IEnumerable<string> topicManagementResults;
-
         /// <summary>
         /// Initializes a new instance of the <see cref="TopicManagementResponse"/> class.
         /// </summary>
         /// <param name="topicManagementResults">The results from the response produced by FCM topic management operations.</param>
         public TopicManagementResponse(List<string> topicManagementResults)
         {
-            this.topicManagementResults = topicManagementResults;
+            if (topicManagementResults == null)
+            {
+                throw new ArgumentNullException("Topic management response list is null");
+            }
+
+            if (topicManagementResults.Count() == 0)
+            {
+                throw new ArgumentException("Topic management response list is empty");
+            }
 
             var resultErrors = new List<ErrorInfo>();
             for (var i = 0; i < topicManagementResults.Count(); i++)