Prevent ArgumentNullException in refresh request (#963)

src/Microsoft.SqlTools.ServiceLayer/ObjectExplorer/ObjectExplorerService.cs

@@ -243,7 +243,7 @@ namespace Microsoft.SqlTools.ServiceLayer.ObjectExplorer
 
                 string uri = refreshParams.SessionId;
                 ObjectExplorerSession session = null;
-                if (!sessionMap.TryGetValue(uri, out session))
+                if (string.IsNullOrEmpty(uri) || !sessionMap.TryGetValue(uri, out session))
                 {
                     Logger.Write(TraceEventType.Verbose, $"Cannot expand object explorer node. Couldn't find session for uri. {uri} ");
                     await serviceHost.SendEvent(ExpandCompleteNotification.Type, new ExpandResponse

test/Microsoft.SqlTools.ServiceLayer.UnitTests/ObjectExplorer/ObjectExplorerServiceTests.cs

@@ -207,6 +207,26 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.ObjectExplorer
                 }));
         }
 
+        [Fact]
+        public async Task RefreshNodeGivenNullSessionShouldReturnEmptyList()
+        {
+            RefreshParams expandParams = new RefreshParams()
+            {
+                SessionId = null,
+                NodePath = "Any path"
+            };
+
+            // when expanding
+            // then expect the nodes are server children 
+            await RunAndVerify<bool, ExpandResponse>(
+                test: (requestContext) => CallServiceRefresh(expandParams, requestContext),
+                verify: (actual =>
+                {
+                    Assert.Equal(actual.SessionId, expandParams.SessionId);
+                    Assert.Null(actual.Nodes);
+                }));
+        }
+
         [Fact]
         public async Task CloseSessionGivenInvalidSessionShouldReturnEmptyList()
         {
@@ -385,7 +405,7 @@ namespace Microsoft.SqlTools.ServiceLayer.UnitTests.ObjectExplorer
             serviceHostMock.AddEventHandling(CreateSessionCompleteNotification.Type, (et, p) => result = p);
 
             await service.HandleCreateSessionRequest(connectionDetails, context);
-            Task task =  service.CreateSessionTask;
+            Task task = service.CreateSessionTask;
             if (task != null)
             {
                 await task;