Fix issue 1665: prevent sending null error message

src/dotnet/commands/dotnet-projectmodel-server/Messengers/CompilerOptionsMessenger.cs

@@ -18,13 +18,13 @@ namespace Microsoft.DotNet.ProjectModel.Server.Messengers
                    Equals(local.CompilerOptions, remote.CompilerOptions);
         }
 
-        protected override object CreatePayload(ProjectContextSnapshot local)
+        protected override void SendPayload(ProjectContextSnapshot local, Action<object> send)
         {
-            return new CompilationOptionsMessage
+            send(new CompilationOptionsMessage
             {
                 Framework = local.TargetFramework.ToPayload(),
                 Options = local.CompilerOptions
-            };
+            });
         }
 
         protected override void SetValue(ProjectContextSnapshot local, ProjectContextSnapshot remote)

src/dotnet/commands/dotnet-projectmodel-server/Messengers/DependenciesMessenger.cs

@@ -22,14 +22,14 @@ namespace Microsoft.DotNet.ProjectModel.Server.Messengers
                    Enumerable.SequenceEqual(local.Dependencies, remote.Dependencies);
         }
 
-        protected override object CreatePayload(ProjectContextSnapshot local)
+        protected override void SendPayload(ProjectContextSnapshot local, Action<object> send)
         {
-            return new DependenciesMessage
+            send(new DependenciesMessage
             {
                 Framework = local.TargetFramework.ToPayload(),
                 RootDependency = local.RootDependency,
                 Dependencies = local.Dependencies
-            };
+            });
         }
 
         protected override void SetValue(ProjectContextSnapshot local, ProjectContextSnapshot remote)

src/dotnet/commands/dotnet-projectmodel-server/Messengers/DependencyDiagnosticsMessenger.cs

@@ -19,11 +19,11 @@ namespace Microsoft.DotNet.ProjectModel.Server.Messengers
                    Enumerable.SequenceEqual(local.DependencyDiagnostics, remote.DependencyDiagnostics);
         }
 
-        protected override object CreatePayload(ProjectContextSnapshot local)
+        protected override void SendPayload(ProjectContextSnapshot local, Action<object> send)
         {
-            return new DiagnosticsListMessage(
+            send(new DiagnosticsListMessage(
                 local.DependencyDiagnostics,
-                local.TargetFramework?.ToPayload());
+                local.TargetFramework?.ToPayload()));
         }
 
         protected override void SetValue(ProjectContextSnapshot local, ProjectContextSnapshot remote)

src/dotnet/commands/dotnet-projectmodel-server/Messengers/GlobalErrorMessenger.cs

@@ -16,9 +16,12 @@ namespace Microsoft.DotNet.ProjectModel.Server.Messengers
             return remote != null && Equals(local.GlobalErrorMessage, remote.GlobalErrorMessage);
         }
 
-        protected override object CreatePayload(ProjectSnapshot local)
+        protected override void SendPayload(ProjectSnapshot local, Action<object> send)
         {
-            return local.GlobalErrorMessage;
+            if (local.GlobalErrorMessage != null)
+            {
+                send(local.GlobalErrorMessage);
+            }
         }
 
         protected override void SetValue(ProjectSnapshot local, ProjectSnapshot remote)

src/dotnet/commands/dotnet-projectmodel-server/Messengers/Messenger.cs

@@ -22,16 +22,13 @@ namespace Microsoft.DotNet.ProjectModel.Server.Messengers
         {
             if (!CheckDifference(local, remote))
             {
-                var payload = CreatePayload(local);
+                SendPayload(local, payload => _transmit(MessageType, payload));
-
-                _transmit(MessageType, payload);
-
                 SetValue(local, remote);
             }
         }
 
         protected abstract void SetValue(T local, T remote);        
-        protected abstract object CreatePayload(T local);
+        protected abstract void SendPayload(T local, Action<object> send);
         protected abstract bool CheckDifference(T local, T remote);
     }
 }

src/dotnet/commands/dotnet-projectmodel-server/Messengers/ProjectDiagnosticsMessenger.cs

@@ -19,9 +19,9 @@ namespace Microsoft.DotNet.ProjectModel.Server.Messengers
                    Enumerable.SequenceEqual(local.ProjectDiagnostics, remote.ProjectDiagnostics);
         }
 
-        protected override object CreatePayload(ProjectSnapshot local)
+        protected override void SendPayload(ProjectSnapshot local, Action<object> send)
         {
-            return new DiagnosticsListMessage(local.ProjectDiagnostics);
+            send(new DiagnosticsListMessage(local.ProjectDiagnostics));
         }
 
         protected override void SetValue(ProjectSnapshot local, ProjectSnapshot remote)

src/dotnet/commands/dotnet-projectmodel-server/Messengers/ProjectInformationMessenger.cs

@@ -26,9 +26,9 @@ namespace Microsoft.DotNet.ProjectModel.Server.Messengers
                    Enumerable.SequenceEqual(local.ProjectSearchPaths, remote.ProjectSearchPaths);
         }
 
-        protected override object CreatePayload(ProjectSnapshot local)
+        protected override void SendPayload(ProjectSnapshot local, Action<object> send)
         {
-            return new ProjectInformationMessage(local.Project, local.GlobalJsonPath, local.ProjectSearchPaths);
+            send(new ProjectInformationMessage(local.Project, local.GlobalJsonPath, local.ProjectSearchPaths));
         }
 
         protected override void SetValue(ProjectSnapshot local, ProjectSnapshot remote)

src/dotnet/commands/dotnet-projectmodel-server/Messengers/ReferencesMessenger.cs

@@ -22,14 +22,14 @@ namespace Microsoft.DotNet.ProjectModel.Server.Messengers
                    Enumerable.SequenceEqual(local.ProjectReferences, remote.ProjectReferences);
         }
 
-        protected override object CreatePayload(ProjectContextSnapshot local)
+        protected override void SendPayload(ProjectContextSnapshot local, Action<object> send)
         {
-            return new ReferencesMessage
+            send(new ReferencesMessage
             {
                 Framework = local.TargetFramework.ToPayload(),
                 ProjectReferences = local.ProjectReferences,
                 FileReferences = local.FileReferences
-            };
+            });
         }
 
         protected override void SetValue(ProjectContextSnapshot local, ProjectContextSnapshot remote)

src/dotnet/commands/dotnet-projectmodel-server/Messengers/SourcesMessenger.cs

@@ -20,14 +20,14 @@ namespace Microsoft.DotNet.ProjectModel.Server.Messengers
                    Enumerable.SequenceEqual(local.SourceFiles, remote.SourceFiles);
         }
 
-        protected override object CreatePayload(ProjectContextSnapshot local)
+        protected override void SendPayload(ProjectContextSnapshot local, Action<object> send)
         {
-            return new SourcesMessage
+            send(new SourcesMessage
             {
                 Framework = local.TargetFramework.ToPayload(),
                 Files = local.SourceFiles,
                 GeneratedFiles = new Dictionary<string, string>()
-            };
+            });
         }
 
         protected override void SetValue(ProjectContextSnapshot local, ProjectContextSnapshot remote)