Handle no language clients being available

[mrward]

VS Mac 8.3 - with the language client installed and no other language clients available - opening a document fails since the LanguageServerServices could not be initialized.

System.NullReferenceException: Object reference not set to an instance of an object
  at MonoDevelop.LanguageServer.Client.LanguageClientDocumentControllerExtension.SupportsController (MonoDevelop.Ide.Gui.Documents.DocumentController controller) [0x00024] in <1d4a1beee10f462593b47f9dfd0f117f>:0 
  at MonoDevelop.Ide.Gui.Documents.DocumentController.InitializeExtensionChain () [0x001f4] in /Users/vsts/agent/2.155.1/work/1/s/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Gui.Documents/DocumentController.cs:823 
  at MonoDevelop.Ide.Gui.Documents.DocumentController.Initialize (MonoDevelop.Ide.Gui.Documents.ModelDescriptor modelDescriptor, MonoDevelop.Core.Properties status) [0x000d6] in /Users/vsts/agent/2.155.1/work/1/s/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Gui.Documents/DocumentController.cs:424 
  at MonoDevelop.Ide.Gui.Documents.DocumentManager.LoadFile (MonoDevelop.Core.FilePath fileName, MonoDevelop.Core.ProgressMonitor monitor, MonoDevelop.Ide.Gui.Documents.DocumentControllerDescription binding, MonoDevelop.Projects.WorkspaceObject project, MonoDevelop.Ide.Gui.FileOpenInformation fileInfo) [0x0023c] in /Users/vsts/agent/2.155.1/work/1/s/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Gui.Documents/DocumentManager.cs:506 

Unable to initialize LanguageServerServices.
Microsoft.VisualStudio.Composition.CompositionFailedException: Expected 1 export(s) with contract name "MonoDevelop.LanguageServer.Client.LanguageClientProvider" but found 0 after applying applicable constraints.
  at Microsoft.VisualStudio.Composition.ExportProvider.GetExports (Microsoft.VisualStudio.Composition.ImportDefinition importDefinition) [0x001d4] in E:\A\_work\149\s\src\Microsoft.VisualStudio.Composition\ExportProvider.cs:355 
  at Microsoft.VisualStudio.Composition.ExportProvider.GetExports[T,TMetadataView] (System.String contractName, Microsoft.VisualStudio.Composition.ImportCardinality cardinality) [0x000b5] in E:\A\_work\149\s\src\Microsoft.VisualStudio.Composition\ExportProvider.cs:811 
  at Microsoft.VisualStudio.Composition.ExportProvider.GetExport[T,TMetadataView] (System.String contractName) [0x00000] in E:\A\_work\149\s\src\Microsoft.VisualStudio.Composition\ExportProvider.cs:251 
  at Microsoft.VisualStudio.Composition.ExportProvider.GetExport[T] (System.String contractName) [0x00000] in E:\A\_work\149\s\src\Microsoft.VisualStudio.Composition\ExportProvider.cs:241 
  at Microsoft.VisualStudio.Composition.ExportProvider.GetExport[T] () [0x00000] in E:\A\_work\149\s\src\Microsoft.VisualStudio.Composition\ExportProvider.cs:236 
  at Microsoft.VisualStudio.Composition.ExportProvider.GetExportedValue[T] () [0x00000] in E:\A\_work\149\s\src\Microsoft.VisualStudio.Composition\ExportProvider.cs:256 
  at MonoDevelop.Ide.Composition.CompositionManager.GetExportedValue[T] () [0x00000] in /Users/vsts/agent/2.155.1/work/1/s/monodevelop/main/src/core/MonoDevelop.Ide/MonoDevelop.Ide.Composition/CompositionManager.cs:81 
  at MonoDevelop.LanguageServer.Client.LanguageClientServices.Initialize () [0x00006] in <1d4a1beee10f462593b47f9dfd0f117f>:0 
  at MonoDevelop.LanguageServer.Client.LanguageClientServices.EnsureInitialized () [0x00011] in <1d4a1beee10f462593b47f9dfd0f117f>:0